Booksellers keep erotica - both literary and graphic - under lock and key. Not because of taboos, but because true erotica is rare. To be an erotic writer or painter requires a rare combination of originality and skill without which the millions of works focusing on sex fall into the readily available category of pornography. Such collaboration as Brian Nissen and Carlos Fuentes undertake here is a rarity. Certain fundamental affinities made it possible. Nissen is an exuberant draftsman, whose line springs with tremendous verve into the service of humor. Erotic drawings, he says, must have a fundamental humor, otherwise they become morbid. Fuentes matches Nissen there: for in countless works, he has demonstrated the irrepressible will to call the bluff of the professors, to excavate language for its brightest ambiguities in which all humor lies, and to exonerate that medium of all lovers of language, the pun. Puns, both visual and graphic, are always the secret bearers of analogy, which, as Fuentes says here, is one side of the modern artist's nostalgia, the other side of which is the temptation of diversity. Both of these artists have known and kept in balance the two.