Who's Who in Wagner by Phillip Hudson (Macmillan: $14.98) is a dictionary of historical figures, mythological sources, characters in the convoluted music dramas, symbols and narrative themes. It offers plot synopses for readers who want to know why those sopranos are pretending to swim in the Rhine or why that fellow with the loud voice is plucking that sword from a tree trunk. The British author--the jacket blurb identifies him as a "journalist, broadcaster and lifelong Wagner devotee"--goes to odd extremes in his critical judgments: Melchior was "the leading Heldentenor of the century," but Solti is merely a "British conductor born in Hungary." In many cases the biographical information is more then sketchy. Friedelind Wagner, the composer's outspoken grand-daughter and a brave anti-Nazi, doesn't even rate an entry of her own, but Hudson does devote 20 lines to Grane, Bruennhilde's horse. Still, this is a handy handbook for the pseudo-sophisticate who doesn't know anything about the Bard of Bayreuth and is afraid to ask.