I was just reading a thread to which I wanted to contribute, and have now lost it. It was how to pare down a cookbook collection which is outgrowing the space which has been allocated to it. I do this exercise every few years, and now is the ideal time to do so.
When I need to do so, I typically start on January 1st with a pad of Post-It notes that I put beside my cookbook collection. Throughout the year whenever I consult a cookbook for ideas or for a recipe, I put a Post-It inside the front cover, and put a pencil strike on it. At year's end, I go through my books and review those that I've not consulted throughout the year and select amongst those which to give away. Of course, even if I've not opened a Julia Child, or the Joy of Cooking, these will not be given away. But, at least taking a closer look at those that I've not even looked at for a year gives me a better idea as to which cookbooks I might be able to part with. (BTW, I do the same with clothing...items which have not been worn for a year often make the trip to the Goodwill bin!)