Hi. Look at these two examples:All book is hard to understand/All books are hard to understand. Are these sentences correct? If so, is this reason right? If we have a singular noun afterall, we use singular verb, otherwise we use plural verb. Is this rule the same forall of? Thanks.

All books are hard to understand
is correct.

You cannot use ‘all’ with a singular noun because ‘all’ is plural.

“All book is hard to understand” is wrong. It would have to be “All (of) the book is hard to understand”. (As is typical, other determiners, such as “this”, “my”, etc., etc., can substitute for “the”.)

Otherwise, your reasoning is correct: plural verb with plural noun; singular verb with singular noun, for both “all” and “all of”.