In Vietnam, Finn, the protagonist of this crude but emotionally convincing debut, learned ""in a twisted sort of way, everything he'll ever need to know about life, about living."" Yet for much of the novel, Finn is only just getting by on liquor and drugs in the netherworld known as Paranoid Park in San Francisco--a place where homeless people, many of them vets (or ""shopping cart soldiers""), gather to forget their common past. Much of the novel is narrated in the voice of Finn's spirit, a kind of chorus (italic type distinguishes it from the more straightforward third-person narrator) that tries to explain Finn's post-traumatic terrors. The split between the two voices reveals a fault line in Finn himself and suggests that whatever acts he has witnessed or committed have torn his soul apart. Finn's character is further fragmented by his heritage: because he was born in Scotland to Scottish parents, he can't even claim patriotism to account for his part in the war. The story of the down-and-out veteran is well-trodden ground, but the novel's details and the verve of the prose are credible enough to keep the reader interested. Although Finn's spirit-voice occasionally dips into the maudlin register of a Capra-esque guardian angel, Mulligan recovers his tone when he evokes the desolation of Finn's bleak corporeal existence. The sincerity of that tone will engage many readers who have an interest in the war and its least-sung survivors. (Sept.) FYI: The author, himself a Scottish veteran of the Vietnam war, was homeless for 10 years. Maxine Hong Kingston taught a veteran's writing workshop where she encouraged Mulligan to write this novel.