I save the Michilen Red Guides every time I go to France. The long line of red on the bookshelf reminds me of many good times. And, I have heard, the Red guides are actually valuable should one wish to sell them.

As for other guide books, I just toss 'em after a couple of years. They are ephemeria. Good for the moment and then useless (with a few exceptions). I wouldn't try to donate them because they really have no value to travelers once they are out of date.