Plaintiveness and spunk are Janis Ian's most endearing characteristics. ""I write about 40 songs a year and all are copyrighted,"" she says. ""Last year was really great--I wrote 119."" So far only two of this fructifying teenager's compositions have been ""catchy"" enough to pierce the over-30 barrier: ""Society's Child,"" an appealing dirge about an aborted interracial romance (""Baby. I'm only society's child/ When we're older things may change/ But for now this is the way they must remain""), and ""Shady Acres,"" an almost bizarre bit of whimsey ironically lamenting the generation gap (""Send your parents to Shady Acres. . .""). The maudlin quality echoes the exquisite nostalgia of Simon and Garfunkel, while the kicky directness, as well as the surrealist tinting, owes its inspiration chiefly to Bob Dylan, Father Goose of the Rock Menagerie. In Who Really Cares, a collection of poems, Miss Ian displays a variety of moods mostly in the notational mode, mixing Salingeresque themes (lack of communication, the phoniness-of-it-all) with post-Beatnik friskiness. Basically, Miss Ian is too commonsensical (and too ""adjusted"") to be the spokesman for the sort of amorphous yearnings usually associated with her peers, yet too honest, too much the mistress of her idiosyncrasies, to turn out commercial sentiments. Miss Ian has her own world, and her own grievances (both social and romantic), but she does not appear to wallow in identity crises or obscure probings, as witness ""Poem for the Young Idealist,"" probably her last word on finky psychology: ""teacher said do you/ hate your mommy? i said i/love her and she said you're/ lying you don't really/ mean that/ do you?/ i said i'm sorry i/ love her and she said i was/ very/ sick."" Slight but engaging.