Here is a truism about studying: Sometimes we have to read things we don't understand. This happens to me more than I would like. However, in most cases it is a problem in me rather than the book. After all, if the author understood what he wrote and what he wrote is, in principle, comprehensible (and these facts are not guaranteed), then I should be able to understand it.
If you are struggling to understand something, here are 8 tips:
#1 Read wide and shallow. Skim the writing. What is the topic or problem? What is the rough idea or view? Once you know these basic facts you have enough to locate the writing. Locating the writing requires going outside it to other works of refernce. The best place to start is an introduction, either a historical one or a survey of the topic. Historical introductions will locate the writing in a chronology of thought. A topical survey will locate the view of the writer within a taxonomy of paradigmatic theories. Try to categorize the writing within a tradition and in contrast to the other most common views.
#2 Read narrow and deep. Sometimes reading a writing that is complex is overwhelming. Every argument seems obscure. Don't try to get to the bottom of every argument. Just pick one and figure it out. Draw pictures, diagrams, read it outloud, turn it into a story, whatever works! If you can get one argument down, it is very likely the others will follow.
#3 Read inside then outside for outline. Can you outline the writing? Try to write out the conclusion and main premises in "textbook argument form." Can you name the form of the argument? Is it inductive or deductive or a bit of both? Once you think you've got it, check with others. By others, I mean academic reviews in journals. Do they outline the writing the same way you do? Where do you think they/you went wrong?
#4 Read submissively. This is tricky if you naturally don't agree with the writing. But it is worth it. If you are ultimately going to disagree with the author, then you really ought to submit to him first. The better you state his argument in his favor the better the refutation will be. This requires a good sympathetic state of mind so you're best off doing this when you are in a good mood.
#5 Read rebelliously. This is the other side of the coin - fight everything, every statement. Try to argue against the writing, taking every assumption to task. I sometimes pretend to be a militant atheist when reading a book written by a Christian. This way lies discovery. As you try to refute everything you will discover novel ways to approach the material.
N.B. If you can read both submissively and rebelliously, all the better. This is best done in two readings but is possible in one if you can switch back and forth.
#7 Read footnotes. This is especially true for journal articles but counts for books as well. Writers often have to trim down their work to fit the required word count. Footnotes are the place they put things they wish they could have placed in the main body.
#8 Read cited materials. This might be a little time consuming but it is worth picking a couple of the more frequently cited sources and reading them. When you are completely stuck and have no idea what the article is about sources are vital. Read as many as it takes to comprehend the writing. Sometimes writers assume that the reader is familiar with certain definitions and does not elaborate. Philosophers like in-jokes and copy each other's entities for thought experiments. Tibbles, the cat, Descartes-minus, and a tachistoscope feature in more than one philosophical writing and often assume that the reader gets the reference. We're not always in on the joke, but sources usually reveal the originator.