I don't know where you are - if you're in the US, your local Barnes & Noble, or Amazon
.com will have a large selection of books on horse care. It might not be a bad idea to try some of the Pony Club manuals (even if you are not in Pony Club) because they are pretty comprehensive. I think when I bought my horse I had three or four guides that I got from B&N that were helpful. Sign up for an account at thehorse.com. Accounts are free, and you can get access to some potentially very useful articles about veterinary and related issues, and get e-mails about stuff that is relevant to your horse. If you want a book about riding, I think "Centered Riding" by Sally Swift is pure genius.