Although this is not a Silverlight book, Silverlight is so closely related to WPF that it makes sense to include at least a brief introduction to Silverlight here.
Briefly stated, Silverlight is a version of WPF that is intended to run in web browsers. The basic idea is that Silverlight applications should be able to run anywhere on any browser (via a browser plug-in) on any operating system. This idea of running anywhere was emphasized by Silverlight's original name WPF/E, where the E stood for "everywhere."
To make running Silverlight in a browser possible, it uses a smaller version of the .NET Framework than the version used by WPF. That means some of the features you've been using in WPF are unavailable in Silverlight. The differences between Silverlight and WPF have been changing with each new release of Silverlight and WPF so I'm not even going to try to list them here. Any such list would almost certainly be out-of-date before this book was printed.
A few places you can look to learn about differences between WPF and different versions of Silverlight include:
Silverlight Overview —
Silverlight Differences on Windows and the Macintosh —
A short forum discussion about the differences between WPF and Silverlight —
Programmatic Differences Between Silverlight and WPF — A Whitepaper