Keep them if you're a collector or are interested at having old rules at hand to compare with current rules for discussions etc.
I see old Codexes pop up on ebay regularly. Not sure how well they sell tho.
Other than that you're probably stuck with just throwing them away if the above options aren't for you.
Under what circumstances would you not give them to a charity shop or concern?
(although - maybe that's a UK thing only)
They may not have the up to date rules, but most of these books are full of lore, brilliant artwork etc. and will be wanted by someone.
Well we don't have such things here. Also I don't really like the idea of letting people who have little money spend it on some outdated rules just so they get all excited and then learn what they bought isn't exactly useable anymore.
Lore and artwork can be looked up on the internet for the most part to be fair.
Ah that's interesting to know about Germany! UK has so many charity shops (almost more than normal shops in some places, which I suppose shows a bad state of affairs).
I would argue though that an old GW book (or any rulebook actually) never actually becomes unusable. As a child I continued to play Rogue Trader despite 2nd edition being released (I couldn't afford all the new boxset and rules). I have just been having a lot of fun with my nephew playing 1st edition Space Hulk, he was absolutely enraptured by the 80s artwork and very basic minis.
Not everything has to be new and shrink-wrapped to be enjoyed, as much as GW (or any other games producer, for that matter) would like to persuade you otherwise.