Bootstrap Resources

General

• Bootstrap CSS
• Bootstrap Components
• boostrap-sass
• Bootstrap Helpers

Grid System

• The Subtle Magic Behind Why the Bootstrap 3 Grid Works
• Bootstrap: Advanced Grid Tricks - You really should be defining your own classes (Will Schenk)

Customising

• List of Bootstrap Less Variables
• A Sass With Bootstrap Workflow (Pivotal Labs)

Using Mixins

• Using Bootstrap as a Mixin Library (Adam Wathan)
• Twitter Bootstrap's Undocumented Mixins - A Reference for Bootstrap 3