The Horse Trailer Owner's Manual by Leigh Goodison Grieve is a great resource for beginner and advanced horse owners alike. From hitches to wiring to winterizing, and everything in between, this book will assist the trailer owner in choosing, maintaining and repairing their horses' means of transport. There's even a chapter on troubleshooting and a checklist for buying a used trailer.
I loved that there's a chapter on the tow vehicle. I met many horse owners that pull their heavy trailers with a too small of a truck!
If you are in the market to buy a horse trailer, buy this book first!