I am currently helping to write a series of grading reading books for a publisher in South Korea, but they are looking for more people to join their bank of writers because it's a pretty big programme.
You need to have a background in education (ideally teaching experience) because the books are linked to the UK and US curriculum, and the sentence length/word choice is analysed so the book is tailored to the right level.
It's good steady work - a contract is issued per book so you set the deadline in agreement with the publisher. Some are original works that tie in with the curriculum and others are rewrites of classics or traditional stories.
If anyone thinks it might suit them, feel free to WWmail me and I'll give you more info.