Here is all the documentation for Nikola. Note that the documents may not always be up-to-date. If you want to read the documentation for the current GitHub master, check out Nikola on ReadTheDocs.

The Handbook
Aimed at those wanting to use Nikola to build a site.
The file
This is the default configuration file for the latest version of Nikola. It contains some useful documentation not mentioned in the manual, and can be used for people upgrading to compare with their existing config files.
Creating Custom Pages
How to do custom pages for special purposes, with special templates and styling.
Theming Tutorial, Theming reference, Template variables
Useful if you want to create or customize a theme.
Extending Nikola and Nikola Internals
Aimed at those wanting to add new functionality.
Creating a Site that is not a blog
Not everything is a blog!
Path handlers documentation
Documentation for path handlers (link:// scheme).
Using Social Buttons
Replacing AddThis with other solutions.
Automating Nikola rebuilds with Travis CI
Blogging anywhere with Nikola, Travis CI and GitHub Pages.
Tutorial by Tim van der Linden
Goes from installing to developing a custom theme.
Tutorial en español de Diego Leonardo Puente
Instalación y deployment a GitHub
Tutorial by RCA (in Chinese)
Covers installation and getting started
Wordpress migration tutorial by Niko Wenselowski (in German)
He wrote most of the Wordpress importer so he knows ;-)
Moving from Octopress to Nikola
Since we don't have an automated tool...