Study this book; read a word then ponder on it...Do not just read, memorise or imitate, but so that you realize the principle from within your own heart study hard to absorb these things into your body. — Miyamoto Musashi, The Book of Five Rings
Reading books is task-based. It’s focused on finishing.
Studying books is progress-based and it is focused on advancing understanding. That is, that the goal is not to understand, but to better understand.
Reading is often linear in nature. You start at the front of the book and you read to the end (or as far as you can before you grow tired of it).
Studying involves more jumping around. You start first with identifying what you don’t understand then you find books or articles that address those topics. From there you don’t just dive in and read from start to finish. You again identify which parts of the article or the book address what you want to know. Indexes and table of contents are very helpful here. If you think in terms of algorithms, this is more like a sort and search approach rather than just searching in randomly sorted material. That is, you sort the order in which you read the chapters and sections of a book in terms of the things you want to understand from the book. Doing this upfront work often saves a ton of time when all is said and done. It also leads to more enjoyable reading as you aren’t laboring through material you don’t care about in order to get to the good parts.
Most books seem to be written with the linear reader in mind, but this is only how it is presented. If you think about the writing process, the mind tends to jump around to what it finds exciting. Most authors only have fragments of the ideas they want to present in their head. It is the editors who force the linear nature of books.
Organization does matter, I’m not arguing that it doesn’t. But the natural way we absorb information is closer to how we generate the information. It’s a process more of filling in the gaps than it is progressing through a linear series of steps.
I think the act of reading is also why many people find mathematics hard. If instead they followed Musashi's advice to "read a word then ponder on it", then the the process would not be as difficult. Certain things just aren't linear and some things aren't meant to be finished. Progress and better are all you can hope for.