The best way to remember more is to read more. I don't mean rereading the same book. I mean go read four more books on the American Revolution. By the last one, you'll have assimilated the information. :)

In the meantime, though, the best way I can think of is the--don't run away!--time-honored, student-abhorred strategy of taking notes.

But don't worry, you're not going to be writing down all the nitty gritty details (dates, people, etc). That's the information that just...comes along. Instead, you're going to aim for writing down the big ideas and turns that the book takes.

Glorious Cause is more of a narrative take on the American Revolution than a traditional academic book that makes an argument about a more focused topic, so it does lend itself a bit more to the people-dates kind of record. Still, you can get around the dry stuff somewhat.

I've talked before on AH about "how to read an academic book" for academic purposes. I'll modify that a little. :)

(1) Before you read the book, read a couple reviews of the book if possible. Not Goodreads type stuff, but really meaty reviews. Unfortunately, a lot of these are going to be behind paywalls still. If you don't have and can't get access (one way or another), a couple potential sources are H-Net and blogs. Generally I find these by searching Google for [title] [author] review and then skimming past all the Amazon/Goodreads-style results.

Why a book review? Good question. Academic book reviews aren't a traditional "will you enjoy this book or not." Instead, they serve as a great guide to (a) what the book is actually saying (b) what to pay attention to as you read. I generally jot down a couple notes from the reviews as well.

In a case like this, hopefully the reviews will help tell you what kind of perspective on the A.R. the author takes--is it elitist, a revolution "of the people, by the people, for the people," etc. This is important because it gives you mental categories in which to organize the information you're reading, rather than just memorizing dates.

(2) Read the book's introduction. For a narrative history like this, it can hopefully fill in some of the gaps if you can't find reviews. A really good intro in a traditional academic book will actually be where you take more of your notes--it's the "Tell you what I'm going to say" section.

(3) When you finish each chapter or each section, take a few minutes. Talk about what you read, out loud. You can explain to a very patient friend, or you can shout at Siri. The best way to learn something, after all, is to teach it. This is because you are taking the time to process the information, to organize it in a way that makes sense.

(4) Once you've done that, take some notes on it. You can transcribe what you've talked at your phone if you took that option, but it's probably better for recall if you type it up afresh. Sometimes it helps to glance back at the book. Again, you're not looking for class-caliber, multiple-choice-test style notes. Just enough to remind yourself what happened in the chapter.

Don't make this a burden. I read books for my comprehensive exams where I ended up with 25 pages of date-people-action notes, and books where I ended up with 1.5 pages of "this chapter says X" notes. Guess what I remember more?

I write 95% of my AH answers off my notes on books and articles I've read. Information recall, today, is as much about knowing where and how to find the info you don't know as it is about what you know off the top of your head.

Taking notes in the manner I described should not give you automatic recall of things like casualty rates at specific battles or how a particular general's perspective changed over the course of the war. But it will give you the knowledge of where to find that out. With some time, opportunities to look that information up, and some further reading, eventually you'll find that you can dredge it up at your weekly pub quiz. :)

Try handwriting notes. I keep a reading notebook, mostly as a memory device, and occasionally use it to look things up. There are a few studies that support handwriting as a memory aid. And fun fact: I've had to consult my reading notebook a total of TWICE for reddit reasons, once was to look up the exact ticket prices for French opera, the other to look up the names of the 4 non-binary genders identified in Talmudic medicine. I'm serious, I remember what I read that well. And I'm not a good memorizer or anything, it took me about a year to commit my husband's cell phone number to my head for safety reasons, and a lot of hard work to memorize even my SSN. I also took all my notes longhand in college/grad school, I never took a laptop to class unless I was told to, because I'm an awful student and I knew I'd be playing BubbleSpinner or whatever, but also because I practice the handwriting trick. It seems like hard work but truth is I'm really lazy, far too lazy to study much in college, or to read a book twice.

A few of my tricks/methods:

Context

As u/sunagainstgold says, if you read more you'll retain more. But I find that the best way to accomplish this is if I, while reading Book B, regularly pause to reflect how it relates to Book A I read previously.

Where do the authors disagree? Where do they agree? What stuff did B make me think differently about? What event did I apparently not understand in A? Where do I think B is missing something important that A did point out?

Not only does this make it easier to remember, but it will also help keep the books distinct in your mind, which might be important if you later want to use them as sources, to look something up in them, or if you'll be taking exams based on them.

Notes

u/caffarelli says to take handwritten notes, which is good, but I do it slightly differently.

If I'm serious about retaining what I read, I start by taking handwritten bullet-point notes. You know the type.

    - Compare to other books you've read
        ^ Where do they agree?
        ^ Where do they disagree? 
            * Who do you think is correct?
        ^ Where do the subjects overlap?
    - Take notes

Much more efficient than writing stuff out full-length, and much easier to study later. Write page numbers down too. It's not much effort now, and you'll thank yourself later if you ever want to use the book as a source, or look something up.

But I don't just summarise the information in the book. I also add my own thoughts and ideas as I write. (Clearly marked as such, mind.) I'll write down what I think is really cool, or really surprising. I'll write down questions that I want to look into later. If I think the writer is being a bloody idiot, or needlessly obtuse, I write that down too, in language that I might not care to repeat on exams. This makes note-taking and reading more fun, and also helps memory. I at least remember my own opinions much more clearly than those of other people. It also makes studying the notes much more fun.

The real trick, though, is that I then copy these notes to my computer, and in doing so re-arrange them in an order that makes sense to me. Doing this both improves my notes immensely, making them much more organised, and it also helps memorisation. When I've done this, I hardly need to review the notes again. The process of re-arranging and re-writing them (from handwritten to typed out) sticks them in my memory. (essentially, it's similar to if not quite as good as explaining the stuff to someone else, which the others have mentioned. The advantage, of course, is that you don't need someone to explain it to, which helps if people around you start developing that slightly panicked look upon your approach that says "He's going to talk about history again, isn't he?")

Also, now that they're in my computer files (backed up, of course) I can search through them if I'm looking for info later. Handwritten notes have their advantages, but they also have a way of getting lost. At least if you're me.

However, I have to admit that I only did this level of notetaking in when I was in university, and nowadays for work-related reading. If I read for pleasure, or to expand my general knowledge of topics, I just use the first method. (Placing stuff into context) It works well enough for those purposes.