Ever buy a book while at an event or traveling only to find you already had a copy when you got home?
I attended IGHR [Institute of Genealogy & Historical Research] at Samford. I've been a member of Librarything for awhile but I've never really used it. Amy Giroux, CG, CGL, suggested using it to list your genealogy library. It is stored online and available to you from any computer so you can't forget it. If you are organized you can print out a list of your books before you go. If you aren't you can still print out a list when you get there.
It's free for the first 200 books. If you want to catalog your entire collection you can get an annual membership for $10 or a life membership for $25. You can make your catalog private or public. If it saves you buying one duplicate it is probably worth for that alone.
This is my genealogy library, at least what I have entered so far. I haven't done the local histories or surname books.
Of course, you are not limited to genealogy or any one subject. And there are other uses for the list -- for your insurance should something happen to your collection like fire, flood, tornado, hurricane for example.
I see another use for the catalogs. Perhaps you simply cannot find the book you need, particularly one of those local histories or a long out of print book. Maybe you can go Librarything, see who has the book and, very nicely, ask if they would mind looking it up for you in their copy.