NAKED AT FORTY by Ruth Benjamin. (Horizon: $10.95). This thoughtful, well written and elegantly crafted first novel suffers from a protagonist who is not especially engaging. Nearing 40, Marian Cross Bartolph--small town wife, mother and would-be poet, braves Manhattan alone to attend her first literary convention. She's tentative, tremulous, but so pure of sensibility that she can spot a shallow socialite, man-hating feminist poet, or pedantic professor half a drawing or lecture room away. At the conference, a surprise encounter with professor and childhood friend Myles hold out the key to the riddle of her painful past and blocked poetry. Marian's subsequent voyage of self-discovery unfolds to a graceful and satisfying conclusion.