Don't be put off by any mildly negative reviews you read below. This book is a joy to read. I haven't read any of Holt's other works except The Portable Door (which I ordered as soon as I'd read the first few pages of this one and realised that, no, you may not need to read one as a prelude to the other, but it does help) but, as I had done with that one once I received it, with the luxury of half a day free I read it from cover to cover in one sitting. His writing style is easy without being childish, the jokes are subtle and in context (more situations than jokes), with the effect of making this an amusing fantasy rather than a fantastic comedy. I like the fact Holt doesn't use puns to bring a wry smile, nor obvious slapstick, just a story told with the right amount of cynicysm and depth of character to keep me amused and interested. Think Terry Pratchett without the footnotes and puns, or Robert Rankin without the slightly loopy prose and, to be honest, you're probably nowhere nearer imagining Tom Holt, but I'm no good at comparing authors so you'll have to make do.
If you've read The Portable Door then it's no-brainer that this book should follow. If not, read that then read this. I'm off to hunt through some of his back-catalogue.
Incidentally, four stars rather than five, 'cos it didn't change my life, but who wants a book that does that?