THE COMPLETE GILBERT & SULLIVAN GUIDE by Alan Jefferson (Facts On File: $22.95) is by no means a complete guide to the Savoyard operas. True, the 14 librettos are reproduced entirely, but with no glosses on obscure lines. More annoyingly, Jefferson alludes to possible erotic alternative meanings but unpersuasively offers only two examples as proof. Most of the material in his brief introductions is available elsewhere, except for persistent name-dropping, throw-away allusions to classical composers (Verdi, Mozart, Wagner, Handel, among others). Fortunately, Martyn Green's "Treasury of Gilbert and Sullivan" (Fireside: $14.95), recently re-issued in paperback, offers wonderful commentaries on Savoyard history and staging, plus piano arrangements of more than 100 songs. The only criticism: too few librettos (11) and not every tune!