WiX: A Developer's Guide to Windows Installer XML
Books about Windows Installer are very rare. Some people have requested that Rob Mensching, head of the WiX project, write a book about WiX, but he's too busy working on the WiX toolset itself. So Nick Ramirez has taken on the task and wrote a book about WiX (and it looks like the book also introduces the underlying concepts of Windows Installer along the way).
Nick is an active member of the WiX community, so I assume that the book has quality content (however I haven't read it myself). You can download the table of contents and chapter 1: Getting Started. Another sample chapter is also available: Adding a User Interface. On Amazon you can also take a look at all the other chapters.
The book is available in paper or electronic form from Packt Publishing and (as printed book) from Amazon:
Nick even donates a portion of the proceeds to the WiX team.