Despite what Estlor says, it is possible to run a Warcraft campaign using nothing but the basic Warcraft RPG book, just like it's possible to run a Forgotten Realms campaign using nothing but the FRCS. The basic Warcraft RPG book includes races, classes (both core and prestige), feats, spells, geographical information, adventuring hooks, background information on the Horde and Alliance, etc. Basically, it has everything you need to get a campaign going.
The other Warcraft RPG books add a lot, but they aren't essential for running a Warcraft campaign. The Manual of Monsters (my favorite of the Warcraft books) contains lots of new creatures to throw at your PCs, while the Alliance & Horde Compendium contains some new races, prestige classes, feats, and a set of Mass Combat rules.