When it comes to books, what do you think is the “perfect” amount of pages?

Depends on the book. With some books 5 pages is too much. With others, there can never be enough pages. You never want it to end.

Granting that some books are too long at 200, and some too short at 800, and that a book can be all over for me at page 1: still, to answer your question as I understood it, I’m generally optimistic about the length when I start a novel that’s about 350–400 pages.

For nonfiction, it depends so much on the subject matter. A work in psychology or self-help can deliver its one little message in a few pages and pad it out to 120 or so with anecdotes. A book about Japan in the aftermath of World War II or one that presents the context and analysis for Roger Willliams’ influence on the social and political history of the American colonies is going to take a whole lot longer.