Unfortunately the only books that cover birding sites in Germany are all written in German. This one covers Sankt Goar:
http://www.amazon.de/Vögel-beobacht...r=8-1-fkmr0&keywords=vogel+beobachten+in+west
I don't know of any sites specifically around Sankt Goar, but the Rhine valley is a good area to see Rock and Cirl Buntings. They prefer old vineyards. Both Icterine and Melodious Warblers occur here as well, while Yellow-legged Gull is fairly common on the Rhine itself.
A good resource for getting recent bird sightings is ornitho.de. See
here for recent birds around Sankt Goar.