I enjoyed this. It's a strange style, with lots of "dear reader"-type sections where the narrators address the reader directly. It quite flagrantly mixes fantasy and real history (not usually my cup of tea). The language is often too modern for the time period, but where that often bothers me, it doesn't here. I think it's because of the rather silly—but very charmingly silly—premise and tone. Often humorous fantasy starts taking itself too seriously and loses the humor, but this never does. One reason may be that it isn't exactly humor. It's a good story with a humorous bent and off-beat style. A very nice balance.
Because it never takes itself too seriously, I can overlook the unrealistic bits (like convincing various people of things too easily) and the unnecessary diversions (The great bear? Is that just poking fun at quest stories?)
I like the characters and found their relationships satisfying.
Keep an eye out for my review of My Plain Jane, the second book in the series, a supernatural version of one of my all-time favorites: Jane Eyre