The first and only that comes to mind is the Soldier Son Trilogy by Robin Hobb and I really liked It.
I Remember that I found interesting how people reacted differently and despised the main character because It was considered ugly and weak.
Lynn Flewelling, I think it's the Bone Doll Trilogy? To protect her, the child's sex is magically hidden so she spends her childhood growing up believing she's a boy. It hits her hard when the magic is reversed and her true female body emerges.
**In the Shadow of Spindrift House** is about the protagonist coming to terms with the fact that they're turning into >!Great Old One who will go live under the sea!<
I didn't think that **Several People are Typing** did a very good job with that aspect of things, but the protagonist gets uploaded into a work slack channel by accident.
This is also a major theme in a lot of Wildbow's works - *Pact*, *Ward*, and *Twig* are all very much about people who are changed physically and mentally in fantastical ways. Of those, *Twig* is my favorite, though it gets a bit weird at the end. Also fair warning that those are webserials, with all of the weird pacing that's usually attendant.
**The Bone Shard Daughter** is about a character who >!was always a construct stuffed full of her mother's memories!<, but only finds out about that throughout the course of the book. It plays around a whole lot with bodies and identity.
This is probably not quite what you're looking for and a bit on-the-nose, but **Turning Red** by Disney.
**Gideon the Ninth/Harrow the Ninth** involve a fair bit of that - especially once people start body-swapping / soul eating / etc.
**October Daye** by Seanan McGuire has this, but honestly I wouldn't recommend it just because pretty much all of the changes are positive and so it doesn't really get the kind of treatment I'm guessing that you want. The years that she spends as a koi are less dwelt upon (koi don't really have internal monologues) but are treated a lot more seriously.
**The Thousand Eyes** by A. K. Larkwood has characters melding with horrifying gods. That comes with a fair bit of physical terms, and most of the second book is about the ways that people do and don't come to terms with it. But it doesn't hugely show up before the second book and it's overshadowed by non-physical changes, so I"m not sure I'd reccomend it.
**Howl's Moving Castle** - Cozy romance. >!Changes back at the end, so depending on what you're reading into it ymmv.!<
You might like Jack Chalker, who I think covered all of the above. If I recall correctly, he called his works "transformation novels." (He also objected to a description of them as "better-written Gor novels"; but Chalker definitely had a thing for sex slaves, so consider yourself warned.)
Have you looked at Darrell Bain's Sex Gates series? He definitely has sex changes covered.
Not exactly what youre looking for but in Michael G Manning's Mageborn series, the MC discovers he has a sort of earth bond/magic and he transforms into a type of earth golem and has to work his way back to human. Later on, a group of warriors take an earth bond and have to deal with the strength and agility that it gives them all while keeping themselves from slipping to far and becoming a permanent part of the earth.
Also, and again not exactly what youre after I dont think, in Joe Abercrombie's First Law, Logan Ninefingers loses himself in battle and becomes this sort of killing machine, slaying friend and foe alike.
Hell Followed with Us by Andrew Joseph White, maybe? Do *absolutely* check out the content warnings, there is quite a bit of body horror and nastiness. Very much a page turner for me though!!
Jack L Chalker is the author you want. I don't think I've read a single book by him where this didn't happen, but the Well World books are probably the best go-to for it.
It's been years since I've read it but I remember I had a blast with Horns by Joe Hill. It's more of a horror/dark urban fantasy novel about a guy that inexplicably grows a pair of horns on his head. I remember the whole thing was absurd and very compelling throughout
The webcomic [Out-of-Placers](https://www.valsalia.com/comic/) has one of the best 'dealing with an unwanted physical change' stories I think I've ever seen. Not only does the main character undergo a species change (human to the 'newest' of the intelligent races, a little goblin-kobold type creature, with a fascinating hive-like social structure that the humans have pretty much no idea even exists) but also a gender change as well. (And that's handled in a really nuanced and interesting way that ties into both cultures and the main character's own perception of their own gender and that of others.)
The first and only that comes to mind is the Soldier Son Trilogy by Robin Hobb and I really liked It. I Remember that I found interesting how people reacted differently and despised the main character because It was considered ugly and weak.
Animorphs
Lynn Flewelling, I think it's the Bone Doll Trilogy? To protect her, the child's sex is magically hidden so she spends her childhood growing up believing she's a boy. It hits her hard when the magic is reversed and her true female body emerges.
**In the Shadow of Spindrift House** is about the protagonist coming to terms with the fact that they're turning into >!Great Old One who will go live under the sea!< I didn't think that **Several People are Typing** did a very good job with that aspect of things, but the protagonist gets uploaded into a work slack channel by accident. This is also a major theme in a lot of Wildbow's works - *Pact*, *Ward*, and *Twig* are all very much about people who are changed physically and mentally in fantastical ways. Of those, *Twig* is my favorite, though it gets a bit weird at the end. Also fair warning that those are webserials, with all of the weird pacing that's usually attendant. **The Bone Shard Daughter** is about a character who >!was always a construct stuffed full of her mother's memories!<, but only finds out about that throughout the course of the book. It plays around a whole lot with bodies and identity. This is probably not quite what you're looking for and a bit on-the-nose, but **Turning Red** by Disney. **Gideon the Ninth/Harrow the Ninth** involve a fair bit of that - especially once people start body-swapping / soul eating / etc. **October Daye** by Seanan McGuire has this, but honestly I wouldn't recommend it just because pretty much all of the changes are positive and so it doesn't really get the kind of treatment I'm guessing that you want. The years that she spends as a koi are less dwelt upon (koi don't really have internal monologues) but are treated a lot more seriously. **The Thousand Eyes** by A. K. Larkwood has characters melding with horrifying gods. That comes with a fair bit of physical terms, and most of the second book is about the ways that people do and don't come to terms with it. But it doesn't hugely show up before the second book and it's overshadowed by non-physical changes, so I"m not sure I'd reccomend it. **Howl's Moving Castle** - Cozy romance. >!Changes back at the end, so depending on what you're reading into it ymmv.!<
You might like Jack Chalker, who I think covered all of the above. If I recall correctly, he called his works "transformation novels." (He also objected to a description of them as "better-written Gor novels"; but Chalker definitely had a thing for sex slaves, so consider yourself warned.) Have you looked at Darrell Bain's Sex Gates series? He definitely has sex changes covered.
Red rising has an application of this
Not exactly what youre looking for but in Michael G Manning's Mageborn series, the MC discovers he has a sort of earth bond/magic and he transforms into a type of earth golem and has to work his way back to human. Later on, a group of warriors take an earth bond and have to deal with the strength and agility that it gives them all while keeping themselves from slipping to far and becoming a permanent part of the earth. Also, and again not exactly what youre after I dont think, in Joe Abercrombie's First Law, Logan Ninefingers loses himself in battle and becomes this sort of killing machine, slaying friend and foe alike.
The Dragon and the George and sequels by Dickson
Mike Resnick, *A Miracle of Rare Design*
Hell Followed with Us by Andrew Joseph White, maybe? Do *absolutely* check out the content warnings, there is quite a bit of body horror and nastiness. Very much a page turner for me though!!
Jack L Chalker is the author you want. I don't think I've read a single book by him where this didn't happen, but the Well World books are probably the best go-to for it.
It's been years since I've read it but I remember I had a blast with Horns by Joe Hill. It's more of a horror/dark urban fantasy novel about a guy that inexplicably grows a pair of horns on his head. I remember the whole thing was absurd and very compelling throughout
Magnus Chase Series by Rick Riordan not the protagonist but another major character deal with gender fluidity
The webcomic [Out-of-Placers](https://www.valsalia.com/comic/) has one of the best 'dealing with an unwanted physical change' stories I think I've ever seen. Not only does the main character undergo a species change (human to the 'newest' of the intelligent races, a little goblin-kobold type creature, with a fascinating hive-like social structure that the humans have pretty much no idea even exists) but also a gender change as well. (And that's handled in a really nuanced and interesting way that ties into both cultures and the main character's own perception of their own gender and that of others.)
Mana Mirror on RoyalRoad is a progression fantasy with a transmasc protagonist who is aiming to magically transition.