Okay, here's how I'd tackle this puzzle — I dug around a bit in my mental library and then sketched out the practical steps I'd use if I had the book in hand.
I don’t have a single definitive name to drop because multiple works can share the title 'Mere Mortal' (and small-press or self-published novels often complicate searches). If you’ve got the physical book, the fastest route is the copyright page — that’ll list the author, publisher, ISBN, and usually the publication year. If you only have a partial memory (cover art, a character name, or a line), plug those into Goodreads, WorldCat, or LibraryThing; their search systems are forgiving and often pull up alternate editions and sequels. Amazon and Google Books will also show author pages where sequels are linked.
If none of that helps, note any subtitle or series name — sequels sometimes carry the series title rather than the original book title, so searching 'series: "Mere Mortal"' or the protagonist’s name can reveal the follow-up. I get a little obsessive about this stuff, so if you want, tell me any snippet you remember and I’ll help chase it down with you.