The primary documentation for CodeIgniter is its User Guide, included in the package downloads.
The User Guide can also be read online here
CodeIgniter 3.x
CodeIgniter 3.x is the current version of the framework, under active development.
CodeIgniter 3.x is licensed under the MIT License.
Read the User Guide online Download the User Guide Download the epub format U.G.CodeIgniter 2.x
CodeIgniter 2.x is the legacy version of the framework.
CodeIgniter 2.x is licensed under the EllisLab open source license.
Read the CodeIgniter 2 User Guide online Download the CodeIgniter 2 User GuideNotes
What's inside the User Guide?
The User Guide contains an introduction, tutorial, a number of "how to" guides, and then reference documentation for the components that make up the framework.
Where else can I find the User Guide?
The user guide download is simply the zipped folder from the released version. It is included in the framework download.