first, the quickstart rules are on this page.
http://privateerpress.com/warmachine/the-game
the quickstart rules have a simplified ruleset omitting rules about troops, terrain, and such. they also give you the stat cards for the 4 faction starter boxes to be used in mk.2 (all the ones on shelves were produced with the old rules).
when youve tried a few games with the quickstart rules you are ready for the full prime mk.2 book.
in warmachine you do not need to get your army book at all, the models come packaged with cards for their rules so all you need is prime and a set of cards. the books collect the whole faction's releases from the last several years into a single updated volume and add new stuff but the rules are redundant because of the cards.
unfortunately because of the recent shift to the new rules many old models on the shelves(but not all) still have old cards. faction specific card decks were made to help old players catch up. these are labeled as 2010 mk. 2 decks, do not be mistaken by decks not labeled 2010.