Pat, I've been culling my collection in stages and finally getting to the tail end. A few thoughts..
When I first became interested in food, early on I was working with chefs. I bought the books they were interested in. (Multiple components, tweezers and white linen-type books). Now, I realize that's not my style of cooking. I ditched them all.
I also bought several CIA-instructional books. Upon further inspection, some are written for restaurant volumes, others take smaller portions into consideration. Volume book -- gone.
I made a pile of all the books I've never cooked from. After I got over the separation anxiety, I went back (about a week later) and gave them a hard look. You know how some books are so compelling, they drag you into the kitchen? Are any of those books in this pile? Probably not, but worth another look. If not, out they go.
Next I made a pile of all the books I've cooked from, but had less than spectacular results. I've tried two or more recipes and despite the reviews on Amazon or the Big Name Chef on the cover, my results were lackluster. Then I asked myself...do I really want to invest anymore time or money trying to convince myself that this is a great book? No? Out they go.
From there, I went through another cull, ditching anything I'm no longer inspired by, currently have a solid mastery of, etc. As an example, at this point, I no longer need 7 ice cream books. I picked my two favorites and ditched the rest.
And lastly, a friend of mine gave me some terrific advice. If you really end up missing a book, you can always buy another copy. Surprisingly, this gave me terrific peace of mind. I knew I wanted to purge a large chunk of my collection. Once I got past "Mine! Mine! Mine!" and letting go...knowing I could actually buy another copy if I really wanted, made me dive in much more enthusiastically. So far, I haven't replaced anything.
The other thing that helped...I pictured myself explaining to someone, "I love this book because..." If I was grasping for something to say, I ditched it.
While I'm not completely done, I'm getting much closer to what I envision as a "hit parade" of my favorite books.