This looks interesting–an online class on ELT Reading Materials Design by Marcos Benevides. It seems to focus on adapting a work and creating a graded reader, but the website also says it covers creating original materials as well.
It’s a four week course from September 7th to the 28th for $49 or $75 if you want a certificate. It’s through iTDi and the teacher is Marcos Benevides, who authored Widgets and Whodunit (a course book that I have mentioned before as one I wish I had authored myself). Check out the class website for more information and to register.