Yes, it can be used in several contexts. For instance, you can say 'She has the most books' when discussing who of a group has the greatest number of books. "Of all my friends, Peter has the most books." Another example is, "This is the most books I've ever owned". That is, this is the largest number of books I have ever owned". So yes, 'the most books' can work as a phrase.

Sure, you could say, for example, "The library of Alexandria had the most books of all libraries in it's time."

yes