The books are a good way go. If you take the time to search em out on the internet, you can find some really good prices. Also search Hunting & Fishing Collectibles Magazine, and Decoy Magazine, for their issues on Warin Decoys, and try to get those issues as well.
Best of all is to attend a decoy show, where you can see the decoys and maybe get to touch them. Museums are also good, as are decoy auction catalogs.
I can't afford the decoys, but I try to gather as much info as possible.