In Discworld cosmology, the world is flat and is carried on the backs of four titanic elephants who ride the back of a giant turtle through space. Once there was a fifth elephant, which somehow fell off the turtle's back and into orbit, eventually crashing onto the Disc and splitting the land into continents. Millennia later, its bones of iron and nerves of gold are still being mined in the resource-rich land of Überwald. So is its fat, which produces the best candles, lamp oils and soap on the Discworld.

As Terry Pratchett's 24th Discworld novel begins, Commander Samuel Vimes of the Ankh-Morpork City Watch is pressed into service as an ambassador to Überwald. That land is an unstable collection of fiefdoms ruled by a dwarf king in the underground mines and a variety of vampire and werewolf lords in the frigid forests above. The imminent crowning of a new dwarf ruler will be an important time for diplomacy and trade, neither of which interests Vimes much.

It will also be an important time for intrigue, violence, theft and murder--which are more in Vimes' area of expertise. Even before Vimes leaves town, Überwald's problems spread to Ankh-Morpork, where dwarves are rioting in the streets. In Überwald itself, Vimes faces a hostile reception. Many of the conservative local dwarves think of Ankh-Morpork as an obscenely modern place, where some female dwarves actually use the pronoun "her" instead of "him" and wear leather skirts instead of the traditional leather pants (although they still don't trim their beards). It doesn't help that Vimes' retinue includes Corporal Littlebottom--a liberal dwarf on the cutting edge who wears makeup and covers her formal dress axe with glitter. But the biggest problem is that Vimes is nosy, honest, smart, unable to let crime go unpunished and walking into a dangerous political conspiracy.

Shiny surfaces, hidden depths

Terry Pratchett isn't the only major fantasy writer who churns out a book a year in a popular ongoing series, but he may be the only one consistently fighting off authorial entropy. Rather than degenerating into contractual-obligation clones, the Discworld books are actually improving year by year. The series began as fluffy, funny fantasy, but it has gradually matured without losing sight of its roots. While Pratchett still puts out the occasional silly shaggy-dog story (most recently, the Rincewind book Last Continent), he also produces complicated comedy with heart, soul and wicked edges.

The Fifth Elephant is simultaneously a locked-door mystery, a thriller and a screwball comedy. Pratchett juggles his usual large cast of distinctive personalities for maximum humor without sacrificing drama. In this case, the action is split as Vimes and his companions face complex politics, hair's-breadth escapes and a homicidal maniac while, in their absence, the Ankh-Morpork City Watch disintegrates in the overburdened and paranoid hands of Sergeant Fred Colon. But Vimes' situation still has its share of slapstick and sly referential humor--particularly in the presence of the ubiquitous "Igors," a race of near-identical lisping servitors that cater to Überwald's needs. (Particularly the needs that involve fetching body parts and saying "Yeth, Mithtreth?") And Colon's sorry collapse has its serious points as well. The entire book resonates with sharp one-liners and non-sequiturial, absurdist metaphors even as it presents a series of lessons about racism, sexism, fascism and old-fashioned moral goodness.

Pratchett's greatest gift is that he can pack this much serious complexity into a succession of light jokes and still make it look easy. The Fifth Elephant is a breathless book, and reading it is like skating effortlessly across a highly polished mirror. But Pratchett's surfaces consistently reflect hidden depths, and the combination is probably what keeps millions of readers coming back every year for more.