Anyone unfamiliar with the Tillerman familywritten about in Homecoming, Dicey's Song, the recent Come a Stranger and other titlescan begin with this one, a probing story about a quiet boy who ennobles himself simply by being himself and his seemingly straightforward brother, who both takes and inspires action. James Tillerman is 15 now, and Sammy, 12, although the older boy is small for his age and the younger brother large for his. ""Things were so simple for Sammy, clear and simple,'' Voigt writes, but James's refrain is more complicated, for he begins so many sentences with ``Do you ever wonder . . . ?'' He wants to find their father and understand why the man never claimed them, never wanted to see who they were. What he and Sammy discover, in a dank bar where merchant sailors hang out, is that their father cheats almost everyone he meets: their mother of marriage, fellow sailors of money, his children of company, love, supportand (but only if they let him) even of dignity. A complex story of the ways in which people piece themselves togetherwith or in spite of their backgroundsVoigt's book rings with truth and compassion. Ages 12-up. (September)