I don't know anything about reading science textbooks, but I have to read a lot of math papers/textbooks, and I think this strategy is pretty useful in other areas as well. I speed read the entire book/paper. Probably about 3-4 pages a minute. By the end of if, I understand very little, but I know an overview of the material contained. Then I reread with that big picture in mind and usually I kinda pick up on the important details. If I need to go really in depth, THEN I'll pay attention to proofs and MAYBE to practice problems. Obviously if you do everything in the book you'll really understand the content, but for a 300 page math textbook, that's gonna be a few hundred hours.
