WoWDB

Having this item in your inventory allows you to collect items that can be turned in for tickes once a month at the Darkmoon Faire.  You may get your guide from several NPCs on the island and do not need to pick up a new one each month.

The following items can drop while carrying a Guide:

•  Soothsayer's Runes  - raiding
  [quest: Tools of Divination]
  
  
• Imbued Crystal   - five man dungeons bosses
  [quest: A Curious Crystal]
  
  
• Monstrous Egg  - five man dungeon bosses
  [quest: An Exotic Egg]
  
  
• A Treatise on Strategy  - five man dungeon bosses
  [quest: The Master Strategist]
  
  
• Ornate Weapon  - five man dungeon bosses
  [quest: A Wondrous Weapon]
  
  
• Mysterious Grimoire  - five man dungeon bosses
  [quest: An Intriguing Grimoire]
  
  
• Fallen Adventurer's Journal  - PvP
  [quest: The Captured Journal]
  
  
• Captured Insignia  - PvP
  [quest: The Enemy's Insignia]
  
  
• Banner of the Fallen   - PvP
  [quest: A Captured Banner]

These items will only drop if you have the Guide in your inventory.  If it is a dungeon drop, all players carrying a Guide may roll on the item.  Raid drops are looted by everyone with the Guide.  PvP items are looted individually by clicking the bodies of fallen enemies.  It is important to note that these items will only drop while you are doing level-appropriate content.  You can no longer farm low level bosses to earn particular ones.

The items are Bind on Use and can be traded or purchased off the auction house.

Unfortunately, the guide doesn't seem to work in WoD content like dungeons and raids.  (I don't PvP, so I can't speak to that.)

EDIT:  Confirmed that the item does not function in current expansion content.

The question:

@WarcraftDevs ((Why does Darkmoon Adventurer's Guide not work in WoD content? Never see an artifact drop in instances.))

Reply from @WarcraftDevs (source):

@AliokWOWDB That item is currently being revamped and will make its triumphant return very soon!

I'm not entirely sure what that answer is supposed to mean. As far as I'm aware, the guide works in older content - just not Warlords of Draenor content. And why has nothing been officially said until now? (Why not in the fairly recent Darkmoon Faire blog, too?) The Darkmoon Adventurer's Guide is rather important for accumulating DMF tickets via monthly quest turn-ins.

Allowing an item that silently fails to work for the current expansion's content is bad form, Blizzard. Players should know up-front if there's as big an exception to an item's functionality as exists in this case. There's the blogs, Twitter, Facebook, /r/wow, a slew of fansites... not that hard to communicate!

(Sorry for the rant. My jimmies get rustled by this sort of stuff.)