FWIW someone wrote their PhD thesis on the Iu Mien language: https://opal.latrobe.edu.au/articles/thesis/An_Iu_Mien_gramm... It's not a textbook, but that doesn't mean you can't try to use it as a textbook. For example, there are some thoroughly-analyzed example texts. Also, the acknowledgments section mentions the Iu Mien Literacy Project, which has published a short course on the very basics, as it turns out: https://iumienliteracyprojects.com/resources/ I suspect the 925-page PDF contains some more hints to additional resources like that. :)