Most people don’t have photographic memories, so we must face it. Nonfiction books aren’t there to be memorized verbatim. They’re about concepts, not minute details.
Such details come in countless varieties: names, dates, places, you name it. Also, there are forms pertaining to particular fields. Examples of scientific cases should be treated less seriously than the overall scientific ideas themselves. For example, in a Britannica article on vision across the animal kingdom, I do my best to extract the key principles, using a Macintosh note taking tool. Also, tools like highlighters (to be used only on your own printouts/copies!), are of avail. Unimportant details, just to give examples, are such things as animals that may bear a structure or function to work in their habitat, or mathematical reasoning or equations. My ultimate goal in researching topics of any kind is to obtain a “take-home message” (of course, I’m typically already at home, but you most likely get the metaphor).
And some sources are just not for everybody. Some are too easy for one, others too hard, still others not satisfying. I typically use Britannica (and is wonderfully sufficient for me), though I am not endorsing nor condemning it and I leave it up to you to decide which are the best resources for you.
In any case, you could tediously memorize an entire unit of text, whether a paragraph, chapter, or entire book. But I’d prefer just to understand a book or other source of info yielding a take-home message. While the existing knowledge quantity grows, the capacity of the inquiring mind does not. So don’t be too tenacious about learning. It may backfire.