Depends on the book.

Reference books: I read what I want, when I want. That's the point.

Textbooks: Chapter by chapter. First, I skim the section headings of the chapter, to get a sense of what the chapter is about and how it's developing the ideas and concepts. Then I skim again, but with more attention to the sidenotes, figures, and other supplementary material. Then I go back and skim each paragraph. This is where I start making notes, relating what jumps out at me in each paragraph to the themes established by my previous skims. Then, I go back and read some or all of the chapter paragraphs in detail. Finally, if the book includes quizzes or study prompts, I read those, compare them against my notes, and decide if I need to re-read the chapter to pick up something I may have missed.

Histories other than textbooks (biographies, popular histories, etc.): I read them just like fiction. Beginning to end, as a linear narrative.