Rick Shreves just announced the availability of a new book on Mambo: Mambo Visual Blueprint. This Wiley Visual Blueprint series book is the third on Mambo (following two Packt books: Building Websites with Mambo and Mastering Mambo: E-Commerce, Templates, Module Development, SEO, Security, and Performance. I have not yet read it yet but if you like the "Visual Blueprint" style, and you want to learn about Mambo, you might want to check this book out.
For those of you keeping score, that 3 to 2 (if you count the Robert Deutz's German only Joomla! book), Mambo over Joomla!