Basically, do it actively. Reading the book will not get enough into your head for the vast majority of normal human beings.
Make your own notes from the books, abbreviate them, set and solve problems: basically do "stuff" rather than just read "stuff".
When you are at the point where from a few keywords you can recall most of the textbook content, because you have written and used it all multiple times, then start on the practice tests.
Stuff you get wrong on the practice tests, go back to the books and re-learn using the same active methods.
Then do them again, look for 80%+ each time ideally.
Then go sit the exams.
Passive learning is a poor method, and that sounds like the reason things haven't been sticking with you.
I don't know how your particular school works, but in general, good practice is to learn it first, then use the groundschool as revision sessions.
Good luck.
G