Eu acho que isso é Um Milagre do Design Raro de Mike Resnick, que também foi o assunto da pergunta livro sobre um homem enviado para múltiplos planetas, cirurgicamente alterado para se encaixar com os alienígenas .
O protagonista é Xavier William Lennox, que é um autor de travelogues alienígenas. Como você diz, ele sofre modificações cirúrgicas para fazê-lo parecer com os alienígenas que está pesquisando.
No final do livro, ele é informado:
“I've just come back from the Department's headquarters on Deluros VIII,” she continued. “Our budget has been cut—well, re-directed, actually—and your particular project has lost its funding.” She paused, trying to discern a reaction, but finding none. “It makes very little difference anyway. As I told you after your last surgery, it is the opinion of Dr. Ngoni's staff that you can only survive one more transformation.”
Lennox merely stared at her, as if waiting for her to continue.
“In appreciation for your unique services to the Republic, they have set aside enough money to pay for that transformation, but since we have no assignment for you, you are free to choose any race you want—or to go back to being a Man.”
Sua resposta é:
“Dr. Ngoni says you can choose any oxygen-breathing race except for the Domarians and the Mollutei.”
I KNOW WHAT I WANT.
“Good. What is it?”
WATCH.
He entered a number of commands, and the machine slowly pieced together a holograph in response to them.
“I've never seen such a thing before,” said Nora, studying the image. “It's quite beautiful.” She stared at it intently. “One might almost say awesome. What planet does it live on?”
NONE. Then he added: YET.
“Is it your own creation?” she asked, surprised that she wasn't more surprised.
YOUR CREATION. MY DESIGN.