Too new eh? It's funny how people tend to vastly undervalue something if it's offered for free. You paid for parking on top of it plus did the carrying and provided the boxes, so it's almost like you paid them to take the books.
I think that there should be some criteria for accepting books. I don't believe in censorship, however, I doubt that libraries should take books titled Why We Should Kill All Black People. Mein Kampf would be fine, since its historically important (although I've heard that Hitler wasn't a very good writer).
Also, if the library already has several copies of that book, or if the donated books are very poor quality (i.e. falling apart, missing pages, jelly stains, etc.) then that's a valid reason.
I've never actually parked at a library; I don't even have a license (I should get on that, but my hometown is small and I go to college in Boston where I've never needed a car), and its within walking distance anyway. Your library does seem a little extreme, though.
Yes, there should be evaluation of the books donated. The two things that I think have to be extremist and poor quality (physically) books shouldn't be allowed.
I think a fair way to class something as being 'extremist' would be something inciting violence. A book literally falling apart is obvious. (it'd be rather unfair for someone to have to pay for a book if it broke, if it was breaking already.)
Furthermore, on a library by library basis, having more strict criteria could make sense, especially if they are tight on space.
For example: Refusing books that they already have ample copies. Refusing books that don't suit clientèle. (lets say a library is in a business district, and the majority of people borrowing are adults. they have small children's section, but it's full. They do not have the demand for them.) Refusing factual books that are too old (lets say a 1920's book on science.)
However, I think those rules should only be made because of issues with space etc.
You might be surprised what books some libraries are willing to take - and what books some libraries aren't willing to take. I know of some libraries in the US south that won't stock books about abortion or homosexuality. Yet they'll stock books which promote going back to an age of slavery and books which advocate suppressing the rights of women.
The way I see it, libraries should accept ALL books and let their readers/users decide which ones they want to read. Of course, some libraries do have limited space and there's nothing wrong with selling off books that haven't been checked out in years or maybe even decades or for which a newer version now exists in the collection. Those should be the criteria by which books are sold and it should not be based solely on content or because some library board doesn't "approve" of it.