There are plenty of books that aren't anywhere I've ever found. I like to read about the history of space exploration, a niche topic where there are plenty of pbooks, but even most of the best and most prominent books in the area aren't available as ebooks -- Andrew Chaikin's
A Man on the Moon and Michael Collins'
Carrying the Fire being two examples. There are tons of commercial books in this area that are now OOP, some of them truly outstanding, that I never expect will be available, not to mention tons of NASA-series reports. I agree with Elf's assessment that nonfiction is not widely available unless it's something quite recently published.