Fans of the bestselling Grisham will be pleased to note that he is once more on Firm ground: his latest legal thriller offers a clever, compelling plot coupled with two singular protagonists sure to elicit readers' empathy. Eleven-year-old Mark Sway, taking his kid brother for a smoke behind their Memphis trailer park, witnesses the suicide of a lawyer ``driven crazy'' by a lethal secret. Before he dies, the man confides to Mark where the body of a recently murdered U.S. senator lies buried, and the game's afoot. Trailed by the police, the FBI and assorted Mafia types (the deceased politico was the victim of ``a successful New Orleans street thug''), Mark retains--for one dollar--the services of Reggie Love, a 50ish female lawyer. This uncommon attorney-client relationship adds an affecting, unusually humanistic layer to the novel's tension-filled events. Mark, raised by a divorced mother and wise beyond his years, thinks chiefly in terms of movies and TV; Reggie, a street-smart survivor of an acrimonious divorce, is often unsure whether to hug or slug her precocious client. True to form, Grisham employs just enough foreshadowing to keep the suspense rolling (``Neither of them could know that . . . ''), and propels his action at the requisite breakneck pace. Occasional plot improbabilities and stylistic quibbles--a few fuzzy characterizations; overstatement of already obvious points; Mark's sporadic adult phraseology--will not deter readers from enjoying a rousing read. 950,000 first printing; Literary Guild and Doubleday Book Club main selections; Reader's Digest Condensed Book selection. (Mar.)