Written somewhat like a "Rough Guide", it is based on the principle that Fantasyland is a real place (with Embroidery, Rabbits and Dark Lords and other such arcane subjects) and the important information about the place is included in the book. It's a little bit like early Pratchett in some ways; a good-humoured and affectionate piss-take of fantasy tropes.
It is a) very funny (it's funny because it's True - Diana Wynne Jones knows her stuff of course), and b) very sobering (because one recognises many of the clichés that one liberally sprinkles over any fantasy gaming/fiction endeavour), and c) very enlightening (because by analysing the clichés and pulling them apart it's sometimes possible to find the bits that work while discarding the bits that are overused). It goes as much for fantasy gaming as for fantasy writing I think, and will I hope allow me to either a) avoid or b) play up the clichéd elements as appropriate (because sometimes, it is appropriate).
It has also left me wanting to write two things: first, a fantasy series that uses (literally) every cliché in the book, and second, a fantasy series that subverts every cliché in the book. Can we imagine a fantasy world where witches don't attempt to seduce unkempt strangers who are not lost heirs, while hordes of Barbary Vikings don't drink ale and have good-natured fights using battle-axes, and avoid sacking nunneries from which there won't be one survivor? I'm not sure but I'm suspecting I'll have a lot of fun trying.