In this one, "This is about donated books to our library.", "donated" is acting as an adjective describing books, so it sounds a bit weird. For example, "This is about books to our library", doesn't work either. The problem is that "to our library" part being alone. Grammatically, it's not immediately obvious how the library relates to the books without some word between "books" and "library". Of course, a native speaker would probably guess what you mean in this case. However, you could say "This is about donated books sent to our library.", which solves that problem by replacing the verb. In "This is about books donated to our library.", "donated" is acting as the verb in a clause. This might make more sense if you think about this sentence as a shortened version of "This is about books that were donated to our library", where it's more obviously part of a subordinate clause. You can also say "This is about donated books.", because there's no "to our library" part to confuse anything. "This is about books donated." would sound off to a native speaker, who would assume that you meant to say "donated books". However, you could say "This is about books that were donated."