Firs, off, this isn't a novel, any more than The Fellowship of the Ring is. Without any acknowledgement of the fact on the cover of my paperback copy, this is the 400-odd page Beginning of an Open-Ended Series. As such, it resolves a bunch of questions (spoiler alert: mostly the answer is either "Magic!" or "Time travel!") but leaves most plot threads dangling for the next book.
Structurally, the book is mostly a set of dungeon crawls. The parties of viewpoint characters run from one monster battle to another, learning or gaining very little along the way, until they happen to converge on the final Boss Battle.
Along the way, they are menaced by giant snakes, vicious teddy bears, giant bookworms, wolves, a cannibal, some more cannibals, water ghosts, gummy frogmen, wraiths, killer moths, a polar bear, an ostrich, a wild boar, a tiger-skin rug, bloodthirsty cherubs, and miscellaneous sewer bugs. Interspersed with these scenes of peril are long sections in which the exhausted, terrified, homesick, wounded characters engage each other in flirtatious banter and witty put-downs.
It's all inventive and entertaining enough, but the arbitrariness of the perils thrown at the characters, and the constant bouncing between drawing-room wit and gruesome gore has become rather exhausting by the end.