The public believes that all donations are good and that every library would happily welcome and rejoice at whatever pile of outdated, dusty, boring junk they found in their great-aunt's basement. As most librarians known, the public is wrong about this (and many other library-related things).
Let's start with the most common myth, donations aren't free. You (the great unwashed public) give them to me (the humble civil servant) thinking that out of your munificence you are enriching the library at no cost to us. However, we still have to take the book to technical services, add protectorants (the shiny stuff over the dust jacket, the stiff stuff on paperbacks that makes them feel hard), add labels, add security measures, add barcodes, and put them in our catalog. This can involve the labor of anyone from the technical services clerk to the head cataloger and it can involve materials costing anywhere from $1 to $10. All told your "free book" can cost the library somewhere in the range of $25 (or more) to add (labor+materials).
So when I get your donation, I start staring, inspecting, picking apart that gift horse. It's the same economic decision I make when I decide if I should purchase a book for the library. Will this book pay for itself in circulations? Do we have other copies of this book? Other things by this author? On this subject? How well have they circulated?
I guess what I'm really saying is that no, we don't want your 1999 edition of the Writer's Market, I have the 2010 edition on the shelf and only one person has even touched it in the last two months.