Short novel; first person narrative of a man in an empty white cell. Spoiler ahead.

[Louisa]

Spoilers Below:

This novel came out about 10 years ago. I believe the cover was mostly white and very simple. The story is very philosophical as the character is narrating his life or a version of his life to the reader. I believe the setting is in a plain prison-like cell with a desk and materials for the narrator to write.
I want to say there was something about a horse, but that could be wrong.
In the end, the narrator tells the reader that everything he said was made up and that is the joy of a story. Almost like it was the author breaking character and telling that to the reader. I have wondered about the title of this book for years as I'd like to revisit it but have never been able to recall or search for the title.

[Kris]

Added a spoiler alert to the header.

A guess:
- I'm Not Stiller (1954) by Max Frisch? There are different cover images.
(Google search - site:www.goodreads.com "unreliable narrator" man white prison cell alone)

Near the beginning, what does he say is the reason for being in the prison-like cell?