Depends on what kind of book (academic vs. leisure) and it's vocabulary (scholarly vs. laymen).
Leisure reading (Laymen): 20-40 pages. Example:
Brave story has a mix of some beefy chapters (believe one chapter is 33 pages long) and chump change (3-6 pages).
Leisure reading (Scholarly): 5-10 pages. I don't have a problem with writers who wish to practice their ever-increasing vocabulary arsenal , but I will have a dictionary--or my laptop--next to me reading your book, which will slow me down incredibly.
Academic reading (Scholarly) : 15-20 pages. Get in, say what you need to say, and get out. As fascinating and unique a scholar's research is to their project, I don't need--or really want--to know every detail they have researched
in the actual essay. Put your supplementary readings/sources in the footnotes or the notes section at the end of the chapter/book. I promise you, scholars, I will read it. (No really, I will, because I am skeptical with a lot of your essays and would like to see the books/articles you used when you start those interpretative juices going.)
Academic (Laymen): 30-50: If you are able to present you research in such a way that even the laymen can understand it, I will grant you the honors of reading it even twice in one sit-in. These chapters/articles are rare, but when I do fine them...GOLDEN.