I read this book, maybe 8 years ago or a bit more. It is fantasy, and humor, and I think part of it was set here and part in a low-tech magical world. I believe one of the characters was a math teacher, maybe his (?) chapter titles were math equations? It was quite important that he knew math, it might've been significant in the other world as well. and I think another was studying magic and maybe was commuting back and forth between the worlds for her study and her day job. What I recall most right now was this little poem, in the text of the book.
There's a dragon on the cover of the book/ a dragon on the cover of the book/ It is green and it has scales/ and it is no-where in this tale/ but there's a dragon on the cover of the book!
I genuinely do not recall if there was a dragon on the cover. I don't think there was a dragon in the story, although they were perhaps discussing the possibility when the one character starts singing the poem as a joke (and it was not given any other author but the character).
I have not had much luck searching for it, with keywords, quotes, or snippets. Putting the whole thing in quotes is also not working. I am hoping someone else might remember better. Thank you.