It helps to distinguish between primary texts and secondary literature.
Primary texts I read with a pencil in my hand. I can’t really study a text unless I hold it physically and write on it.
I believe that secondary texts are, unless exceptional, to be consulted rather than read. I use the index (also table of contents, introduction) to find where the author discusses matters that concern me, read those, and return the book to the shelf. Some (few) secondary texts require more extensive reading but I believe that many of us are in the grip of a naive belief that we are somehow obliged to read all of a book once we start! Once you learn to consult books, libraries become an ally in the search for knowledge and not an intimidating enemy!
The 80-20 rule tends to apply to books as much as to anything else. The more you know and the more extensive your experience, the more you can focus on the 20% contribution of any given book.