Allows modders to easily create in-game documentation.
Modpack developers: Support for building your books through JSON files is in the works.
Books are maintained by Guide-API (it registers them, it puts them in it's own creative tab, etc)
All the author does is send us the information and we take care of the rest.
Examples for both will come soon.
If you wish to use this in your project, read here.
Go for it.
As of build 2.1.0-51 for Minecraft version 1.11.2 (released on March 22, 2017), the license has been changed from MMPLv2 to MIT. Any build prior to that still uses MMPLv2 which you can view here.