The first book, Warmachine: Prime was released originally in 2003, with a revised edition,
Warmachine: Prime Remix released in early 2007. All of the subsequent books have been expansions of Prime. Each expansion usually introduces new warcasters, warjacks, units, and solos, as well as new model subtypes - such as cavalry units - that add new strategies and complexities to each faction.
Warmachine: Prime - 2003
Warmachine: Escalation - 2004
Warmachine: Apotheosis - 2005
Warmachine: Superiority - 2006
Warmachine: Prime Remix - 2007
Forces of Warmachine: Pirates of the Broken Coast - 2007
Forces of Warmachine is the name of a series of expansions that will focus on a themed army as opposed to having additions to all of the factions. So far, only one book has been released, although it has been stated that more will be announced in the future.
Warmachine: Legends - 2008
Currently under development is
Forces of Warmachine: Retribution of Scyrah - 2009
Warmachine Mk2 - 2010
with a planned release in January alongside stat cards for all the models in the game. Mk2 is a complete reworking of the game, with every model available changed and/or recosted, alongside the changes to the rules set.
April 2009 saw an open "Field Test" of the Warmachine Mk2 rules, hosted on Privateer Press' website. Players could download a full set of rules, plus rules for every model in the game, and post feedback to Privateer Press through an online field test website.
2010 will bring "Faction Books" for all Warmachine Factions, showcasing all faction models for the four factions plus the Mercenaries. Confirmed in a banner on the Privateer Press website for September 2009 is a new faction, the Retribution of Scyrah.