Well, in addition to the main book, everybody who pledges for at least the PDF will get a copy of the (first? One can only hope) PDF supplement, The Devil's Dozen:
". . . an illustrated PDF of 13 new villains, monsters, and NPCs dreamed up by Larry and written up by Steve. We thought you might enjoy a sneak peek at a couple of the ideas Larry's already sent Steve:
El Maldición, the Haunted Gun: This old, intricately engraved, gold-inlaid Colt 1911 chambered in .38 Super seems to possess a strange sentience, and to bring misfortune and disaster to those who dare to wield it."
That right there tells you tells you quite a bit about both the MHI setting, and Larry's writing style. If you're still on the fence, you may want to give his short story Tanya: Princess of the Elves a try (note that there are technically spoilers for the first two MHI novels in there).
But wait, there's more - there's also two exclusive MHI unit patches available. One of them can be added onto any pledge for at least the physical book, for an additional $10 - honestly, I feel that's rather steep, but the patch is very cool: