CraftBook is a multiplayer plugin for Minecraft that adds several features such as bridges, gates, cauldrons, integrated circuits, minecart controls, and much more.
The plugin was created by sk89q and later received development help from several others. The plugin was originally for hMod only, but when the server mod bukkit came out, sk89q joined the bukkit team and stopped support for hMod. A few months later, the official CraftBook was finally updated again, but was only for bukkit.
During the time CraftBook stopped being updated, the unofficial CraftBook extra was made. CraftBook extra was started from a fork of the last hMod code for CraftBook. Originally it was created just to make sure CraftBook worked with the latest version of Minecraft, but later gained additional features.
For information on the official CraftBook, you can visit the official site here: http://craftbook.sk89q.com/wiki/Main_Page