YA Fantasy book With a character who is missing parts of their fingers

[Kaitlynne]

We're looking for this for a patron at my library, this is what they remember:

Patron said they checked it out maybe 8 years ago. It's a fantasy novel/series, he thought probably a YA series. He thinks the word "flame" or "fire" is in the title or the series name. The main character was missing parts of their fingers and chews on the, what he called, knuckle stubs and towards the end this character goes crazy. There is something about a tower or a witch's tower. The only other thing was there are dwarfs in it and they invented cannons... That's all I got. Maybe someone has a guess. It's not "Shades' Children" by Garth Nix. Any guesses are appreciated.

[Kate]

Kaitlynne ~~
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Good luck!

[Allison]

Going off of a very dim memory, this sounds like it might be Fire Lord by Cheryl J Franklin.

[Zev]

Could it be Beneath the Sugar Sky" by Seanan McGuire, of the Wayward Children series? There's similar themes.