Kirsanov believes the market needs both vector and raster applications. At the pixel level, you want raster. Vector programs are a bit more abstract and conceptual.
Why the book when Kirsanov is helping write the online documentation? Because the documentation is a reference, and a book provides his ideas, philosophies of problem solving, and comments about the issues of drawing and design separate from the details of Inkscape itself. Drawing is a complicated subject, and guidelines help users more in some situations than tool explanations. And with this book, you get the thoughts of one of the program's primary developers, always interesting and useful.