Cheaper by the Dozen (Published 2003)

SET THIS HOUSE IN ORDER

A Romance of Souls.

By Matt Ruff.

479 pp. New York:

HarperCollins Publishers. $25.95.

Readers of Matt Ruff's exuberant, population-packed earlier novels will be surprised to hear that his third has a relatively tiny cast, at least on the surface. On the other hand, the star-crossed, parent-wrecked principals of ''Set This House in Order'' both have multiple personality disorder, so another character is always in the offing. These extra personalities are most often allies, though some need to be treated like ''rude houseguests,'' and there's usually a destructive individual aching to get out.

The condition that Andrew Gage and Penny (Mouse) Driver share would be a liability in most places, but not necessarily at the virtual-reality start-up outside Seattle where they labor inside a windowless warehouse under separate tents (a new roof isn't in the budget) along with two perpetually overheated programmers from Alaska. (''As you can see, we're pretty informal here -- a little too informal, sometimes,'' the Reality Factory's owner, Julie Sivik, admits when she introduces Penny. ''This nudist is Dennis Manciple. And Mr. Pouty over there is his brother Irwin.'')

Andrew, the company's wistful ''creative consultant'' and Ruff's most winning creation, has no past -- and an overwhelming past. He is 28, and he is only 2. The discrepancy, once Andrew explains it, is obvious: ''Souls only age when they're in control of the body.'' When he was 3, Andy Gage's soul was destroyed by his abusive stepfather, Horace Rollins, and over the years other personalities emerged, 100 or so eventually existing in a dark space in the child's head. ''In the center of the room was a column of bright light, and any soul that entered or was pulled into the light found itself outside, in Andy Gage's body, with no memory of how it had gotten there or what had happened since the last time it was out.'' (Ruff doesn't overplay the dissociation hand, but Andrew always refers to ''Andy Gage,'' ''the body,'' ''the stepfather.'' The person who might have protected everyone, however, is ''our mother.'')

Andy Gage's people include Adam, an exceptionally acute adolescent who ''upsets the others'' by reading Playboy; the very proper Aunt Sam; Jake, a timid 5-year-old; and Seferis, who materializes when there's physical danger. Ruff excels at the changes in bearing, speech, even appearance when someone new blows in. This may sound fantastic, but he makes them utterly convincing. In fact, he sometimes has to remind us that Andrew isn't the voice of reason we think he is.