Arduin - the OTHER 3 little brown books! Heck, it even says ORIGINAL EDITION on the cover! This unusual 1981 boxed set from Grimoire Games has all three volumes of this self-contained hyper-weird OD&D mutation. It's all you need to be totally confused while having a blast. Magnifying glass recommended.

If you want to learn more about the contents of the Arduin Grimoire CLICK HERE to read Jeff Rients' excellent series of review posts.

There was a recent post at The Tao of D&D about how low-level humans are unrealistically wimpy in D&D. Well, David Hargave proposed a solution for this way back in 1978 in The Arduin Grimoire Vol. 3. In fact, he devised a whole new hit point system with the goal of more realistically balancing hit points across levels, races, and classes. Said Hargrave:

"People now have a chance to run any character or characters on any expedition they choose without regard to difference in levels of experience. They can have their 1st level warrior stand shoulder to should [sic] with a 10th level lord and hold the gate together! Just as in real life young and inexperienced Warriors accompanied older, more experienced fighters. They fought and died together.
   Yet the higher levels have their own rewards, more (but not grossly so) hit points, better fighting ability and the like."

Here's a simplified distillation of Hargrave's system:

STARTING HP:
Base HP by Race:
  Dwarf:     18
  Elf:          20
  Hobbit:    11
  Human:   14

Base HP Modifiers by Class:
  Fighting classes: +5
  Clerics, elves, and dual classes: +3
  (No modifier for magic users)

Base HP Constitution Modifier:
   +1 HP for every point of Constitution
   +1 HP for every point of Constitution over 12 (= the revised CON bonus)

HP ADVANCEMENT:
  Fighters, thieves, and similar classes:     +1 HP every level
  Clerics and dual classes (including elf):  +1 HP every 2 levels
  Magic users and similar classes:             +1 HP every 3 levels

NOTE: Hargrave originally presented more extensive lists of obscure races, classes, and gender differences. I have simplified things here to be more easily compatible with classic D&D.