Encyclopedia of the Horse (forword by Alexander MacKay-Smith)
The Complete Book of the Horse (forword by Lucinda Prior-Palmer)
Basic Horse Training by Heather Smythe (personal favorite as it goes from halter breaking right through to some fairly advanced dressage moves and how to introduce horses to more exotic jumps)
If you can find any of these on ebay/amazon etc I would reccomend them :) Also the british horse society stage 1 book gives a really good foundation knowledge of everything from feeding to pasture management to safety around the yard. Even if you have no intention of taking the Stage 1 exam, its really good reading for building up knowledge!