I personally feel there are multiple "levels" or "layers" to understanding a book. It's a bit loose, but I'd say technical books have "structure", "content", "detail" and "cohesion".
The structure of the book is how chapters, section, subsections and the index are related to each other. You can learn about the structure by taking a look at the index, reading chapter intros/outros/summaries, and you will get a better feeling for it while reading the book as well.
The content is basically just everything in the book except any meta-content, such as headings, the index and references, etc... So graphs, text, chapters, blablalba
The details of the book are the most relevant, precise, thought-out content, you could alternatively call these "topics". Consider a given chapter of any book, it usually introduces a topic, gives some examples, counterexamples, might tell a story about it, it could add a debate, show a graph, and it might describe sub-topics. The whole chapter is mostly about that topic as a whole anyway. That's the detail.
Lastly, the "cohesion" is how the book's topics are interrelated on a detailed level. For example, what implications does the content introduced in chapter 5 have on chapter 6? And on chapter 4? Some concepts take multiple chapters (or books!) to comprehend as a whole. Sometimes a book needs to introduce some concepts before the bigger picture can be explained. This cohesion, in my mind, is related to the "larger than the sum of its parts" idea.
I guess the idea of cohesion can also be extended to how the book's content and ideas relate to what you already know, or other books you may or may not have read. Again; it's how the topics interrelate.
Different reading styles, writing exercises and re-readings of a book will give expand your understanding in those 4 layers.
For example, skimming (30 min, skip through entire book) will give you a vague but helpful foundational feeling for the structure and cohesion of the book. Likewise, speed-reading will give a reasonable foundational feeling for the content and detail of the book. Writing a mind-map of a chapter, asking questions about paragraphs, summarizing chapters will improve your understanding of the content and topics.
I personally feel that I often don't completely "understand" a book, I believe that if I take the time to work on those 4 layers of understanding on every level of the book, I would.
I strongly strongly strongly recommend taking a look at the book "how to read a book", the concepts I just stated are my own, but the book helped me completely rethink about how I approach reading in general. I used to be afraid of tacking huge books, I feel like I can actually read them with some confidence
#ENDRAMBLE