WHEN SHE was 14, Evelyn Lau ran away from home. Until she was 16, she roosted sometimes with relatives, more often in group and foster homes, and finally in her own quarters in a rooming house. An obsessive writer, she won literary prizes and gave readings while ingesting a daily pharmacopeia and turning tricks to support her habits and herself Runaway: Diary of a Street Kid (Harper & Collins, 341 pages, $5.95 paper) is an edited version of the journal she kept in those two years. The strongest impression the book leaves is of the author's ability to believe whatever serves the emotion of the moment regardless of external reality. She practises with self-destructive vengeance the dissociation common among abused children, though to what extent she was actually abused and to what extent her childhood anguish was a product of her own psychological makeup is not clear. Certainly she brings upon herself most of the miseries she suffers in the book, blaming and betraying people who go far beyond the call of duty to help her. Lau is a writer of tremendous power. Runaway tells a gripping story, and its revelations about the workings of an adolescent's mind are much more illuminating than a whole shelf of howto-parent books.