When my father passed away last year, I went through his library of technical books. I recycled 900 pounds of books.
Most people who saw this happening wanted to give the books to the library, to a church, or hold a yard sale. But my mother understood, when I explained that my dad wouldn't really want some unknowing kid to learn something he'd just have to unlearn. Nobody needs to know the ins and outs of Windows 95 application programming.
I really think that donating outdated books to libraries and such is just a way of passing on the guilt of recycling large volumes to the people who work at the library. Be bold, do it yourself.
Edit: Of course, do something meaningful with the current and timeless books. I came home with a copy of K&R's The C Programming Language and a few others, and I left the full set of The Art of Computer Programming in my dad's library. I explained to my mom how that set represented my father's entire library.