Outside of works produced within fetish circles (shoutout to Demon-Man's illustrated story sets!), the ones that have directly influenced my art the most are probably:

1) "Transformation" by Edmund Plante, which is a sadly little-known horror novel about a newlywed woman who TFs into a weird alien creature. The way the changes progress and how they alter her relationship with her husband is superb; I love that kind of stuff in TF stories and I hope to be able to convey it as well as Plante does in that novel. It's a HUGE hidden gem for weird (maybe body horror) TF fans that I cannot recommend enough! If you like slow, gradual transformations and you dig seeing how they affect the transformee's daily life and relationships, give it a read!
2) The Shrek franchise. No joke hahaha. As far as love TF stories go, there's very few that are as mainstream as Shrek. I loved Fiona's journey when I first saw the movies and, in retrospect, it probably helped shape my TF-related tastes a lot. Hers is an "ugly" TF that nonetheless gives her a happy ending through self-acceptance, a loving relationship and a new (more shameless) identity. Those are all things that I love and whose origin I think that I can trace, at least partially, back to Shrek!

There are surely many more works that have influenced me (many that I probably can't even realize atm) but those are the two that are the most clear to me right now! Thanks for your question!

P.S. Your question lended itself more to a written answer but I'd love to draw something "Transformation" or Shrek-related in the future so I might revisit this one for a drawn answer or follow-up in the future!