I too have some great old books that I need to move out including a copy of Art Flick's Master Fly Tying guide that includes pics of a young Dave Whitlock. Like Scott I have read them all, some several times and now it is someone else's turn.
I got into the habit of throwing a book or two in my vehicle when I went fishing. My plan was to pass them along to random folks, who were new to the sport, that I met on the water. One of those pay it forward deals.
Seems the biggest majority of fly fishing books are instructional based, and after awhile, they all start reading the same.
My favorites I've kept, but I've been on a trend these past few winter months with the classic novels that I've never had time to read in the past and no regrets.