The methods are documented in html generated by doxygen.
Click on SdFat.html in the unzipped SdFatBeta20120825 directory and explore the Classes tab.
Most of the important methods are illustrated in the examples in the SdFat/examples folder.
There are also examples in the SdFatBeta20120825/extras/exampleV1 folder. These are older but demonstrate rewrite, seek, remove, and other basic methods.