I'd also recommend "The Definitive Guide to Windows Installer", I think it is really beginner friendly as it gives you an deep knowledge of how Windows Installer works in under 300 pages. I'd rather a nice short book that is straight to the point, rather than a 600 page monster with lots of fluff and examples and information that I'll never refer to again. For me, it helped me to understand some of the "magic" that was going on with InstallShield projects, and I'll be using it as a reference if we decide to migrate to WiX in the future (it's planned, just need to find the time between releases to do it!)
I'd also recommend Rob Mensching's blog at http://www.robmensching.com/. Reading through his old blog "When setup isn't just xcopy" is a very good (and interesting) way to get a handle on some of the more advanced functionality in Windows Installer. (Strangely enough, I beleive I saw a couple of answers from Rob on here last week regarding WiX questions.)
