Unless Dan Jones is going to storm your house with a pop quiz, you don't need to remember everything. You don't have to absorb everything. Because you are right here

Reading should be an enjoyable pastime and not some overcomplicated mental gymnastic feat.

One would hope that no author would wish for the reader to work themselves into stress that you have to space it to protect yourself. It also won't help you absorb anything if you are stressed out. There is no need to be ashamed or feel obligated to the author if it is causing you stress but first I think you are coming at this from the wrong angle and that seems to be the main cause of the stress.

You are not going to absorb everything. That isn't on you, that is normal. You are not lazy, stupid, lacking in attention or anything else because of this. It is perfectly fine to not have absorbed everything as that is the case for most people. People don't pick up a book and at the end have every fact memorized. If you have enjoyed yourself, learnt things and picked up the key themes, the key ideas then that is a success.

You don't need to be remembering the name of every battle, person, or date. If you are getting confused about the timeline, see if the book has one and if it doesn't, note down a few key dates that you need (or have a quick glance at wiki when needed). If you come across a figure or a reference to something earlier and you are struggling to remember who/what that is about and it is causing a problem? Check the index (or in the Templer's case, perhaps look at the appendix sections if my google search went right) or if you need to, make a note you can quickly check upon for next time.

Just be careful that it doesn't become every little thing getting noted down. Just where not being able to remember is otherwise causing confusion or hampering your ability to understand the wider lesson.

It is not you. Do not have expectations that you will absorb everything because you won't and trying to will mean you won't enjoy the book and you won't learn. Putting yourself under unnecessary strain to meet an impossible goal. Yes, authors put time and effort but I hope they want people reading to enjoy and learn which you can't do if it is causing you such stress.

When you read a fantasy/romance//murder mystery or whatever genre of fiction you like (book, tv, podcast), do you expect to remember every single detail? Or just the key details with some memorable scenes? Enough for the next chapter or a sequel to make sense? When you get history via other means (you mention audio-books and videos), can you recite every minute of it or again, a case of you know the general gist? Reading a history book for the first time is not going to be different, you are not going to be able to recall each and every detail.

I am an amateur, self-taught in my era of expertise. I certainly can not read a book, however vivid the writing, and remember all the details afterwards, each and every name, person, and date. What my mind will do is (hopefully) remember the key arguments and know that this is where I need to head for future reference.

A lot of it is also that I have enjoyed the era for a long time. I remember dates and people because well I have read a lot (and also played video games about the era) so each time, a little bit more detail drills into my head. You hear/read a date so many times, it becomes second nature. A person and a battle keep popping up, that will stick but over time and repetition, not immediately.

So yes, absorb things passively as you put it. That is fine. Read another book? If you want to really look at the era being discussed (or any particular era) then yes. History about any subject is not reading one book and you have the topic down as there is a lot to cover with so much happening, humans are complex creatures and historians will come at things from different angles to each other. If you like the era and want to really get into it, you will read more than one book to build more and more understanding and to assemble more of the pieces of the puzzle.

You can always come back and re-read Dan Jon's book again sometime but it might be that, at this stage, you should leave the work alone and do something else. So when you do come back, you have had a break and a chance to wind down so you are not stressing again when you pick the Templars back up.

If you find that, even with a more relaxed approach, you are still struggling then I would advise stopping with the book. For whatever reason, sometimes a work just won't land and keep you absorbed. It might be that you read other works and then find this one clicks as it has built a platform. Or it might just be it isn't the work for you at all.

There is no shame in that and if you find "it is interesting but I find I can't quite keep engaged" then it isn't you or the author, it just isn't working. It is worth considering why it isn't working as has been suggested to help in the future but don't turn it against yourself. You are perfectly within your rights to pull out and look to find another book that will keep you engaged. The author did their best but no writer is going to land with every member of the audience while your time is also valuable and trying to force your way through does not help you.

In addition to what others have said, u/Dongzhou3kingdoms in particular, I find it very helpful to read books that are "adjacent" to something I already know something about.

For example, if you read a book about the history of Japan and then a book about the Templars, there will be no overlap and you'll have to remember all the names and dates and places from scratch. (Or rather, you probably won't remember.)

But if your previous book was about the history of medieval France, there will be some overlap. If you had previously read something general about the crusades there will be a lot of overlap. I find it helps me immensely if I have at least some moments of realisation like "Ah, they're talking about that guy from the other book." (The downside of course is that if you rely on this too much, you can get stuck in a rut where you only read about one topic. This is why I often look for books that are adjacent to something I've read before: related but not covering the same subject. But even then, sometimes you got to bite the bullet and read something completely unknown.)

This background knowledge doesn't even have to come from other history books. You can watch "Kingdom of Heaven" where the villain is a templar and then pay attention when the actual history book shows how that film got things wrong. (Fun fact: back in university, I found strategy games much more helpful than history books for gaining a basic understanding of the geography of medieval kingdoms and place names. The games are terrible history, of course, but they allow you to experience something about the geography that for me sticks in my memory much better than just looking at a map.) You could read a historical novel first, and then an actual history book about the same period, etc.

When all that fails and I am reading something and the author mentions something that sounds interesting but I can't place, or a name I think I should remember but don't... well, that's what wikipedia is for. I stop reading and look it up. Wikipedia cannot be relied on for gaining an accurate understanding of history, but it's wonderful for checking some date or looking up how person X related to person Y. Can't do it all the time or your pace will crash, but if something is mentioned repeatedly or seems important (or interesting) it helps a lot.

With a paper book it's easier than with an audio-book, I imagine. (How do you spell that name?) And it's also easier to flip back and forth, to skim over something and go back when it turns out to be important. I don't much listen to audio-books, so I don't have any specific advice for those, I'm afraid.

Finally: most history books are organised by topic per chapter, and it's often quite possible to skip one that doesn't appeal. Maybe you had heard something about their downfall when King Philip IV accused them of heresy and were really looking forward to finding out more. Why not skip to that part then? Sure, you'll miss background and maybe get more confused about names, but you can look those up. You can always go back later.

So to summarise:

• It's much easier to keep track if you already know something to begin with
• If you don't, it helps to start with something short and simple and general. It'll probably get a lot of stuff wrong, but that's okay. You can always refine your understanding later.
• A frame of reference doesn't have to come from history books alone. A film won't teach you history, but it may let you put a face to a name and that helps a lot.
• You can always look something up
• And if the book is still being a pain, you can skip it in whole or in part.

You're not alone. I often feel the same way when I'm failing to finish some text that I've set my mind upon. There are a number of well-regarded, historiographically important books that I tried and failed to read, and I spent longer than I should beating my head against them on te assumption that some sacred truth that could only be learned therein would be lost to me if I failed. But the truth is that I spent far too long fretting about finishing a book that, frankly, I just wasn't that into.

There are thousands upon thousands upon thousands of books on history and from the past to explore. Some will click with you, some won't. Some will be worth the slog, some won't. Follow what interests you and let your drive to learn develop along what lines it will.

During the middle third of this year I barely got any reading done because I was in exactly your position. I had an idea of what I should be reading and focused on that to the extent of what I could be reading. Read the things that grip you, and if a book isn't working for you then don't feel bad about giving up on it. You can always revisit! Read a little bit of a bunch of books to see what clicks. Despite most of my historical knowledge being tied up in Western Europe, I absolutely devoured a book that uses the pretext of talking about the fastest boat run down the Grand Canyon as an excuse to talk about the entire history of that wondrous place (The Emerald Mile) and just finished another focusing on a mining disaster in the San Juan Mountains (River of Lost Souls). Neither one of these is a "history book" per se, but I'm so glad that I spent my time reading them rather than frittering and wasting the hours in an offhand way. There was something about those two books that resonated with me, while other books that are arguably of more historical import (whatever that means), were left unread.

And that's the real important point here: give a bit of thought to the why. What about this book is failing to keep your interest? What about that book immediately gripped you? Was it the prose? The structure? The subject? The argument? Failure at any one of these things can make a book not work for you, just as a spectacular success at any one can make it click. Think on the why when you reach the end of your time with a book, and that will help you seek out the texts that will really light the fire in your mind going forward.