I'm astonished at the recommendations. None of them are remotely like Sir Terry. Not least because most of them get very same-y after 2-3 books, which TP does not. Holt, Rankin, Anthony (a one-trick pony who has had an astonishing number of books published, all things considered), Asprin, et al.

The gods only know why RB mentions Thomas Covenant; I presume MandolinPaul wants humorous fantsy, which that is not. Mervyn Peake is weird (also not humorous in any conventional sense), but many like him, even if Richard doesn't. If you try Eddison (also not humorous), do so in the knowledge that he is a writer from the 20's / 30's, so the style is very different.

The only author mentined who, in my opinion, operates in TP's off-kilter sideways sphere is Douglas Adams. Which isn't to deny the merits of some of those mentioned. But, in my experience, humorous fantasy palls very quickly. One or two books may work; the only author that seems to be able to avoid beating the style to death after that is TP.