SOLVED. Fantasy: Boy enters magical realm, is transformed into adult, becomes a knight. [s]

[Samuel]

Read around 2016, but don't really know when it was published (guessing sometime after 2000).

The novel is framed as a letter from the main character to his brother, who's still in the normal world.

A boy stumbles into a magical world, meets a nymph-type character who transforms him into a physically powerful adult man, takes him as a lover, and then sets him loose in a medieval type world.

RANDOM PLOT POINT: He doesn't use a sword for most of the story, instead he has a sort of long iron bar / club.

[Rainbowheart]

Is this children's, teen, or adult fiction?

[Samuel]

Adult fiction. Had kind of a Guy Gavriel Kay flavor, but it is not a Kay novel.

[Kris]

Samuel, are there fantastical creatures such as dragons (e.g., The Wizard Knight by Gene Wolfe)?

[Samuel]

YES! That's it, The Wizard Knight! Thank you so much, Kris!

[Kris]

Great. Glad you found your book, Samuel. It pays to guess. (The first book in the series - The Knight by Gene Wolfe).

I got lucky with a Google search -- site:www.goodreads.com fantasy boy "a knight" medieval world