You need to read this.
To expand a little on what Scott says, when I lost my collection to the fire, I had some rules about replacing it, too. I wasn't going to buy any book that I'd already read. Well, that didn't last long, but I've managed to keep it under control fairly well. I haven't really bought anything that I'd already read just to have a copy of it. Anything I've bought that I've already read, I intend to read again. Also, new books or replacement copies of books I had before but had never read had to be something that I actually intended to read soon, not just "Oh, that looks interesting, I might get around to reading that someday". I was really bad about that before.
Well, you know where this is going. When I buy a book, I still intend to read it soon, but soon has expanded to mean sometime in the next year or so. I'm sure the definition will continue to grow. I already find myself in the bookstore looking at a book and thinking, "Do I have this one?", and I don't know the answer. I don't think it's happened already, but it's possible that I've bought the same book more than once. (Long-time readers of this blog may recall that I used to have five or six copies of THE BAMBOO BOMB by James Dark. I don't see that one around anymore. But I've already read it, so by rule I couldn't buy it again if I came across a copy. Well, maybe for old time's sake.)
What it boils down to, of course, is that people like me -- and probably many of you -- can find a way to rationalize buying almost any book. And all I can say is, thank God for tolerant spouses.