Inkscape, A Guide to a Vector Drawing Program, 4th Edition
FLOSS Manuals: Inkscape
An international group of contributors is writing a book licensed under GPL3+ and published at FLOSS Manuals. If you want to help writing it or make a localized version, please subscribe to inkscape-docs@ mailing list and tell us!
Inkscape 0.48 Essentials for Web Designers
Packt Publishing released a book on Inkscape by Bethany Hiitola called "Inkscape 0.48 Essentials for Web Designers". The book covers a wide range of topics from creating impressive web site designs to drawing various graphics elements, such as icons, charts, maps etc. "Inkscape 0.48 Essentials for Web Designers" is available as both hard copy and eBook.
Additionally, there is a great website on SVG (which is a W3C standard which Inkscape relies on for storing its documents), called Learn SVG.