Inkscape, A Guide to a Vector Drawing Program, 4th Edition
By Tavmjong Bah. Available online (http://tavmjong.free.fr/INKSCAPE/MANUAL/html/index.php, from inside Inkscape: Help->Inkscape Manual) or as a printed book/e-book (from your favorite on-line bookstore or direct from Prentice Hall).
FLOSS Manuals: Inkscape
An international group of contributors is writing a new book licensed under GPL3+ and published at FLOSS Manuals. Printed version will soon be available from Lulu. 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.