Zykon TheLich said:
EDIT:
Also, I'm not particularly familiar with warmachine as a game, I guess Prime is the main rule book but how do Colossals, Wrath and the various Forces of X books fit in? Is it necessary to buy them to use certain models or are they just fluff?
Well, Prime/Primal are the basic rulebooks for Warmachine and Hordes respectively, including the rules for how to play the game, as well as the rules for the most basic units and warjacks/beasts for each of the factions (3 Warcasters/locks per faction, 6-4 Warjacks/beasts, 5 units and a solo), with Colossals, Wrath and the like working sort of like 40k's Cities of Death, Planetstrike and similar, adding rules for new Warcaster/locks, 'jacks/beasts, units, solos, siege engines, and Colossals/Gargantuans.
The "Forces of X" books are essentially compilations of the rules from prior books, so it serves the same function as a Codex in 40k, only all the models from prior books and editions are still legal, with their rules and cost updated for the new rules.
Note that all PP models for Hordes and Warmachine come with cards that include all the stats, rules and abilities of the model(s), so with the exception of the basic rulebook, none of the books are necessary to play the newer units. They're still well made, contain more elaborate descriptions of units, and include some pretty decent fluff that advance an actual story, unlike 40k's and WHFB's almost static setting, but it is not necessary to buy them.