* He writes both text and illustrations: He is known for creating both the words and pictures in his books. So, it's hard to distinguish "writing" from "illustrating" in terms of counting his works.
* Collaborations: He has worked with other authors on some books, meaning the writing credit is shared.
* Reissues and different editions: His books have been reprinted, re-released, and updated over time, making it tricky to keep track of every version.
You can find lists of his books online, but they may not be exhaustive. If you're curious about a specific book, you can search for it online to see if it's by Nick Butterworth.