Cameron Moll's 'State of the Mobile Web' series is a first-rate introduction to designing mobile-optimized websites. While
part one of the series talks more about the status quo and the likely future of the mobile Web,
part two takes a more practical approach by discussing various ways how a Web designer can best mobilize an existing or planned website. Two more parts will follow dealing with tips and coding techniques and real-world applications.
If you are looking for other useful mobile Web resources,
this section of the css-discuss Wiki is a good place start with.