Just like most open source communities, Wicket is a global community. And with global communities come books in various languages. Wicket in Action is the third published Wicket book in english, and there has been one in chinese. Today I found the announcement of a german Wicket book which is simply called "Wicket". Its subtitle is "Komponentenbasierte Webanwendungen in Java". Even if you don't speak german, the subtitle is easily translatable. The book is scheduled to be released in Juli 2009 and you'll have to speak or learn german to read it :)
I haven't heard about this book before, but I enjoy the fact that more people think it is worthwhile to write and publish books about the Wicket framework. More books means more community, which means more potential readers for all published books.