Too Many Books?

I saw a book for sale, but wasn't sure if I already had a copy. I had a book database years ago, but it couldn't transfer to new computer, and it went by the wayside. So, I thought it was time to start over. I try to keep to reference books only, and even at that, am up to almost 400, just from the shelves. I still have to catalog the ones in storage. I think it would have been easier to just buy the book - especially since no copy of it has turned up  in my collection.