So I know this is beyond obscure, but there was this book that I read that was about a kid that who would go in his backyard and draw shit that he'd see (fairies, trolls, elves?). And one thing he saw was a person who ate only 1 magical fruit that tasted amazing, but made everything else taste bad. I have absolutely no idea what this series was called and need help. Sorry for the obscurity. They made it into a movie mid 2000s and I still cant find it.
The Spiderwick Chronicles? I've never read or seen the movie, but the wikipedia article makes it sound like a promising guess... Fits the field guide and one character tasted the faeries' food and can no longer eat regular food...
https://en.wikipedia.org/wiki/The_Spiderwick_Chronicles