In my personal experience things tend to be there whether or not it's part of your active memory. However, even if it's not, something one of my professors engrained in us was that history is more than simply memorizing factoids, there are plenty of reference books with that information the more important aspect is being able to contextualize the facts, being able to conceptualize it, and knowing how to research a topic. If you're struggling to remember what you're reading, take notes and summarize while you're reading. It's extra work, but it would make the info readily available and the writing could help you to remember it better.

No one can remember everything they’ve read, that’s what notes are for. And bonus: the act of note-taking solidifies information in your memory better than passively reading. Check out u/sunagainstgold’s How to read an academic book for the pro way to read and digest an academic book. This answer by u/commiespaceinvader also goes into the subject of note-taking.