I likewise did not find much about the Slavic god Radagast in the essay “Obscure Languages” in my
Introduction to Elvish, on page 175. What I wrote was only:
Early chroniclers speak of a Slavic god named Radegást ( Радега́ст ). His temple was at Rethra (earlier Radogoszsz) made of wood, and rested on animal horns. Its outside was covered with figures of the gods. Within were kept special insignia and a stabled horse. Some believe that the reference is to a different god altogether and that the name of the town has been erroneously applied to the god.
I no longer know the source of this information other than that I consulted more than a single book.
See
http://en.wikipedia.org/wiki/Radegast_%28god%29 for further information about this god, of which little is known.