How can a writer put something like this in his book? If you think the example is too difficult for the scope of current book don't show it at all! You could also redirect readers to the other resources like: "detailed explanation of this concept can be found at ...". But writing "an elaborate explanation of it is outside the scope of this book" is just stupid and frivolous! I pay for the book to have explanations of difficult and complicated subjects - not to read that it's out of the scope (show external resources!).
Anyway, the book is quite good, but more and more examples in the second half of it are getting blur and need more explanations (maybe better metaphor than a bit artificial "lasagne"?)
I hope the authors will make an effort and upgrade the content of this book to the newest Wicket release 1.4.x that introduced Java generics and refactored many classes (e.g. validators).