the Hordes of the Things rulebook (sans army lists) is also available at no charge from the publisher, Wargames Research Group.
Not only that, but the publisher has uploaded two versions of De Bellis Antiquitatis, the historical game that was the basis for HotT. Those interested in game development can check out the first edition of DBA, and anyone who'd like to playtest it can obtain DBA version 3.0.
You can find plenty of other historical rulesets from WRG on their website. Have fun!