Supplement IV: Gods, Demi-Gods & Heroes (1976)

From the web:

What the authors have done in this volume is to attempt to set down guidelines that will enable you to incorporate a number of various mythologies into your game/campaign. They make no claims that any of this material presented is exhaustive, or even infallible.

The final supplement to the original D&D game, Gods, Demi-Gods and Heroes give the reader exactly what the title says, information on a variety of pantheons for use in gaming. This was the first place that statistics were introduced for deities and led to the ubiquitous “if it has stats we can kill it” mentality that has followed the game for generations. If you are a lover of D&D history, this book is an essential component for a complete collection.
 

Supplement III: Eldritch Wizardry (1976)

 From the web:

The third supplement to the original Dungeons & Dragons first edition. This supplement adds several items related to magic and the use of magic. Druids are introduced as a player character and Psionic combat is also introduced. Many new monsters including Demons and Succubi along with many new magical treasures.

Eldritch Wizardry was the third supplement created for the D&D game. Unlike the first two supplements, this one didn’t offer new lands to adventure in, but rather offered options for players and Dungeon masters to broaden and enrich their existing worlds. Adding in such universally recognized expansions as the druid class, psionic power, and the hierarchy of demons and their lords, no lover of D&D history should be without this landmark supplement. 

Supplement II: Blackmoor (1975)

From the web:

The second supplement to the original Dungeons & Dragons first edition. In addition to new characters classes (the Monk and Assassin), additional monsters, new treasure items, and underwater adventuring, this includes some of the first references to Dave Arneson's "Blackmoor" campaign.

The book also contains a small adventure entitled "The Temple of the Frog".

Blackmoor was the second stop in the creation of D&D campaign settings. Dave Arneson, co-creator of the game that created and industry, made this land for his own personal campaigns and in 1975 showed this secret world to the rapidly growing fan base of the D&D game. It is a must have for any serious collector of D&D history.