I've had issues finding older books. So some of the ones like Sally Swift or George Morris, haven't been converted into the Kindle format.
I do love my Kindle but availability is sometimes frustrating. Plus, some of the older titles have used copies available from marketplace sellers for uber cheap.
But...on the whole its a great thing to have! And with the price drop, I say go for it. Nothing beats have tons of reading on one little device. Althought, word of warning. Do not fall asleep with the Kindle propped on your chest. Your hands will relax and your Kindle will fall on your face.
I am reading Riders right now. Haven't read it in a while, and it is awfully nice not to have to hold that honkin' large book while sitting by the pool! But they don't have Players for Kindle yet so everyone go on and click the "I'd like to read this on Kindle" button!
There are a few other authors that I have checked out, books I probalby wouldn't have tried if I would have had to pay printed prices.
I think I've read nearly every horse-related fiction book available on Kindle... tehe... it is definitely worth it. I went back to reading a "regular" book (something I couldn't find on Kindle) over the past week and it just seems like such a giant pain!
I have read a fair amount of books that were non-fiction equine books, so I know there is a fair amount of availability. You can look for yourself though - Amazon.com and click on the Kindle Store, then search "horse" or "dressage" or whatever you're interested in! Voila, you can see all that Amazon has to offer.