Write him two letters (fan letters), or maybe even three--Mr. Randall has written a whole novel of them, and it could be one of the catchiest items of the season. The letters here include the complaints that N.Y. apartment-dwelling actress Sally Ross gets from neighbors about nocturnal noise, and Sally's constant, chatty communiques to ex-husband Jake whom she really still loves (he married young Heidi in a moment of glandular distraction), and also the increasingly importunate writings of a fan, The Fan, someone called Douglas Breen, who works in a record store. Sally is 46 (Jake says she's being ""Gaborish'), and she has low moments when she feels that she looks like ""what's left in Grant's Tomb"" and needs cheering up from faithful secretary-friend Belle. But, to The Fan, Sally's an ""explosion of adorableness,"" and his communications become more and more insistent, even threatening. Then someone mugs and carves Belle up on a subway platform, and Sally is left quite unprotected. The police write memos to each other about the assailant that they can't identify. Is he perhaps the one who writes and writes and writes and is frightening Sally into a pill-hazed funk? Randall's collection of letterheads--half covert horror, half disarming humor--should sneak right up on you with all the ominous pleasures of reading someone else's mail.