The core book for Swords and Wizardry - a retro clone that takes as it's inspiration the original boxed set and supplements of 70's OD&D.
From the publisher's website:
"Light your torches, don your helmets, and ready your spells... the Swords & Wizardry Core Rules are a "clone" of the original Gary Gygax 1974 fantasy roleplaying game that started it all. With a thriving internet community and tons of support products, Swords & Wizardry is bringing back a lost style of fantasy roleplaying. Forget huge rule books - just play. If you can imagine it, you can do it in Swords & Wizardry. The rules are simple and quick to learn, and they are infinitely flexible."