Yes, anyone may contribute or take books. The more the merrier! If you take a book (or two) from a Library, you do not need to return that exact book. However, in order to keep the Street Library full of good choices for the whole neighborhood, the next time you swing by the Library bring a few books to share.
Street Library book exchanges function on the honor system; everyone contributes to ensure there are always quality books inside.
This way, we all win!