Here is all the documentation for Nikola. Note that it applies to the latest release, so it may not be accurate if you’re using an older version, or are using an unstable version. 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.
Getting Started
How to set up Nikola.
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.
Upgrading to Nikola v8
A blog post detailing all the changes from Nikola v7.
Changelog, Contributors, The MIT License
What’s new, who made this, and the rules for distribution.
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.
A list of some Nikola features.
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.

Community content:

Writing Task Plugins and Controlling plugin task execution order
Tutorials about writing custom task plugins for Nikola.
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...