I've just finished a re-read of Diana Wynne Jones' The Tough Guide to Fantasyland. It's written as a sort of generic travel guide to the worlds found in a lot of modern fantasy, and consists mostly of a series of encyclopedia-style entries describing all the features that the tourist might expect to find during a visit to Fantasyland. There are some unexpected insights - for example, that Fantasyland horses appear to breed by pollination, and that the staple diet seems to be stew.
As I heard the story, Diana Wynne Jones read a lot of generic fantasy to write entries for the Encyclopedia of Fantasy, and she then wrote the Tough Guide to get the experience out of her system. It's all done quite humorously, but the Guide points out many features of fantasy writing that have been repeated somewhat mindlessly by a lot of authors. I'm reasonably confident that at least one author changed various aspects of their fantasy world after reading the Guide.