All is said in the title of my critic AND NOT in the title of the book. "A gentle introduction..." is not only an introduction but a complete review of how to build a web site with Symfony from the beginning to the end. In fact for a beginner, i strongly advice him to begin with this book. It's well written, clear, and concise. Then he won't take a long time to realize that Symfony is a GREAT web framework and that this book is at least the first part of its Bible.
After i recommend to attack the book on advanced aspects of Symfony by the same authors. By the same authors, the 24-days tutorial on the fictive web site jobeet is just an application and to my opinion is a bad choice for a first read (but maybe a good one after "A gentle introduction...". The reference guide is almost useless as it's in fact poorly documented with little comments or examples.