The primary documentation for CodeIgniter is its User Guide, included in the package downloads.
The User Guide can also be read online here
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.