In psychology, this is known as the depth of mental processing. The more you engage with an information (questioning, discussing, paraphrasing etc.), the deeper it becomes engraved in your memory.

I apply an easy on-the-go method with pretty much all the books I read to increase memory retention. Whenever I finish a chapter, I close my eyes and imagine me explaining the contents of the chapter to a good friend of mine. If you know your friends well, you also know what kind of questions they might ask, so feel free to have a discussion with them in your head.

This way, you engage with the material and force your brain to process the text at deeper levels. And it takes only about five minutes per chapter. If you really want to put in the effort and increase retention even more, write down the fictional explanations you have in your head.