Paul, who has published poems in The New Yorker, makes his book debut with this gossamer-thin take of how he and his buddy built a medieval war machine. One day, while hefting a pink-and-white lump of Red Creek quartzite, a wild fancy strikes Paul ``from some dark corner of my mind''; why not make a stone-thrower, a medieval catapult? He talks his chum Harrya devotee of formal Japanese archeryinto playing along. But how to pay for this crazy project? ``I would call it art and apply for a grant.'' So Paul does, wheedling $500 from the amused authorities at California's Headlands Center for the Arts. Library research and model-making ensue, as Harry and Paul puzzle out the winch, crank, bowstring, trigger, and other parts. The two ``artists'' dither, quibble, mull over siege techniques, prowl through junkyards, try out giant springs, and in general have a boyish good time. Finally, after an arduous construction process that offers the book's best laughs, the weary builders, wearing special pigskin gloves, erect their ominous-looking device on the Pacific shore and shoot rocks into the sea. Lots of digressions, ranging from the nature of iron to the history of Paul's father, fetter the narrative, which never builds much forward thrust. A pleasant diversion, then, but hardly a book to crack the castle walls.