Syncing content is a one way street with iPods and that is from your iTunes library to the iPod and NOT the other way around. The only exception to this rule is iTunes purchases, which can be transferred back to iTunes by choosing File -> Transfer Purchase from <name of iPod> with the Shuffle connected to your PC/Mac.
Otherwise, see this excellent user tip from another forum member turingtest2 outlining the different methods and software available to help you copy content from your iPod back to your PC and into iTunes.
Recovering your iTunes library from your iPod or iOS device
B-rock