Manuals and quick references
Inkscape Beginner's Guide
An introductory guide, covering various topics relevant to new users — ranging from installation to making your own font. It is written by the project's contributors, and can be viewed and downloaded at readthedocs.io.
Inkscape, A Guide to a Vector Drawing Program, 4th Edition
Comprehensive manual by Tavmjong Bah. Available online, and linked from inside Inkscape (Help > Inkscape Manual) in English.
Keyboard shortcut reference
A list of keyboard and mouse shortcuts for the latest Inkscape version.
Command line reference
A listing of all available command for using Inkscape from the command line (man page)
Books