WARNING: Please note that this article was published a long time ago. The information contained might be outdated.
Matthias starts by describing how a basic request is handled by Symfony. From the Kernel to the response, everything is described in a detailed and clear way. Then patterns of dependency injection get on stage. Knowing how to create services and how to manage their dependency will make you love this part even if you are not developing with Symfony2. Matthias continues showing how a project should be structured. Also this part is focused on Symfony2 but you'll be able to apply this knowledge to anything. Then the focus turns on configuration and security. The security part is very detailed and it should be "studied" more than once. The last part is titled "Being a Symfony developer" but it makes you understand how important it is to know how a framework (any framework you are using) works so that the code you're writing can be used by others and adapted to different projects.
Personally, I've chosen to buy the book because I had some trouble understanding and memorizing some Symfony2 stuff and I can assure you it really helped me.