Are both sentences below correct? 1. All of the books are expensive. 2. All of the books is expensive.
When all is used with a plural noun, it means every, and the verb agrees:
All the countries were represented at the games.
When all is used with a singular noun, with or without of, it means entire and takes a singular verb:
All the country was in mourning.
To use your examples, you could say "All the books are expensive" and "All the book is in Latin" (which makes sense, but isn't as good as "The entire/whole book is in Latin").