First darktable e-book released
Stefano Fornari, Mario Latronico and Nicholas Manea released a book called "Digital photo development with Darktable". It is available under terms of Creative Commons Attribution -ShareAlike 3.0 Unported license.
The book is across between a workflow guide and a reference, it also provides some essential information on digital imaging for photographers. It doesn't (yet) try to explain every single feature and focuses on fixing composition and black and white photography workflow instead.