Anne, as another book collector, I totally understand how hard it is to purge a book. It's like turning your back on an old friend--and you never know when you might get the urge to go to reread something. When we built our smaller house, I insisted on built-in book shelves in every room, except the bathrooms (although I sort of wonder about that, from time to time!) my builder thought I was nuts, until he came by after we had moved in...
But I recently did purge a good third of my collection when I realized the numbers were seriously getting out of control. For what it's worth, here is how I did it:
First, I told myself that books, like friends, are not all equivalent. Some are dearer to your heart than others, so I classified them as (1) beloved friends who I can't conceive of not having in my life, (2) good friends who I want to keep in my life, (3) former friends whose paths have diverged from mine--probably closer to acquaintances these days, (4) people who I met briefly, enjoyed but made no effort to stay in touch with--sort of like former work colleagues, and (5) passing acquaintances, or people who I didn't feel much affinity with after spending an evening together.
Books that were in the first two categories were put back on the shelves immediately. Books in the 5th category went into the donate bin. Books in the third and fourth category were put in a bunch of bins. I then put the word out to my other book-loving friends and family that these books were up for grabs, as long as they stopped by for a coffee or a drink. It was fun watching them eagerly grab onto so books which had delighted me in the past, recommend others to them, discuss ones we both had read and really ought to give to X, and so on. The pile went down surprisingly fast and I think taught me that many books deserve to be shared instead of hoarded.
I'm still a book collector and will remain so until the end of my days, but my favorites are now easy to see and there is room for new discoveries and friends beside them. I've also got in the habit of sending on a book once I've finished reading it to someone else who I think would enjoy it, with the proviso that it be passed on in due course to someone else. Seems somehow right now not to be stingy about introducing one friend to another.