It's a mistake, and besides impossible, to search for "the first trans novel", however you define any of the terms.

So here's a deliciously incomplete list of novels in English by and about trans people, up to 2010, to highly variable definitions of "trans" and "novel".

Jack Saul, The Sins of the Cities of the Plain (1881)
https://t.co/nMby1oWmV5

Major work of gay/trans porn writing. Probably not by the actual John Saul, sex worker and occasional cross-dresser. Features the infamous Fanny and Stella, who were very trans:

Rachilde, Monsieur Vénus (1884, English tr. 2005)
https://t.co/Ni2jYupQEu

Erotic novel of an abusive BDSM relationship which propels both characters into gender ambiguity, by a writer with strong but complicated male identification and presentation.

Irene Clyde, Beatrice the Sixteenth (1909)
https://t.co/0glyOrWdqk

Speculative fiction about a postgender (but very femme) society, by a lawyer, writer, feminist, pacifist, traitor and war crime apologist who expressed a lifelong desire to be a woman.

John Radclyffe Hall, The Well of Loneliness (1928)
https://t.co/du4qeItptV

Novel of sexual inversion, a sexology category combining sexuality & gender, by an invert who preferred the name John.

Dare you to go on telly claiming this as the 1st trans novel with a major publisher.