What I do for pretty much any book that I intend to learn from is treat it like a lecture; as though I'm going to be quizzed on it afterwards. I have always had issues committing things to long-term memory, but the same methods we use to remember things from lectures work for books too. I typically write out a summary (in a dedicated document on my laptop) of each chapter I read, after i finish reading it, in the most concise manner possible. A few sentences about the narrative, and bullet points about what I want to remember from the chapter. This way, I can return to my notes when I want to remember a book and what it's about and get a solid gist about what's important. Not only does this help refresh my memory, but writing those summaries makes it twice as likely for me to remember the parts in question. Christ knows how hard this was when I read the Old Testament.