They were having a lot of trouble trying to describe Terry's writing in the early days. So you see a lot of such comparisons - very few of them really work, but you have to remember that this was the first book in the series and the publishers wanted people to be intrigued enough to pick the book up and read it.
So mentioning famous humorous writers such as Jerome K Jerome (Three Men in a Boat) and books such as LOTR and Peter Pan are more marketing ploys than anything.
Terry has said that Discworld is a bit like Middle Earth several hundred years after all the wars, when all the various races settle down and try to get along.
Colour of Magic is a somewhat different Discworld book, but it's very worthwhile reading.