Well I actually find having many of these older Edition books and redundant or older books on hand very useful. I have tones of books as well, in fact
FoxtrotUniform and I were discussing this very thing in CB the other day. Tons o' books is a good thing to me :)
...but I digress, I actually use these books as "Safe To Borrow Out" books for newbies and co-workers I am mentoring. I don't like to borrow out current books, and I find that a 2nd Ed book is just as helpful to a FNG as the 3rd Ed, and it also gives them a "preview" to see if they want to go out and buy the latest edition themselves.
"Nothing is sure but death and taxes" I say combine the two and its death to all taxes!