Childhood's End and Guardian Angel
First, information quoted from the letter:
"I've never read "Childhood's End" by Arthur C. Clark, but I have read a short story which I assume is either a prequel for or, perhaps, the original genesis of the novel. This story... deals with a human's attempt to find out about Karellen's true shape (which, in case it's different from the novel, resembles that of a devil). The really interesting thing about it in relation to Xenogears, however, is the fact that the story is named "Guardian Angel." :)" (Vierran)
Karellen in Childhood's End and Karellen/Krelian in Xenogears share very similar temperments. Krelian, in his later form, also handles situations in a similar manner. They both sit in a position of power over the world, and both are present to see humanity reach its peak and transcend its current existence, although their motives in doing so are very different.
The Karellen we see in the book is acting on the orders of a higher power, and he's sad to see humanity advance, when he and his own kind cannot; in the game, Krelian pushes humanity toward its change without mercy, and seeks to transcend his own existence. But he would be unable to do that without the interference of his nanomachines, so in that respect, he is again very similar to Karellen.
One might also say that Krelian is... perhaps the way the Karellen of Childhood's End was able to achieve his dream of ascendence. Krelian does what Karellen could not. That probably has nothing to do with what Takahashi intended, but I think it's a nice sentiment to try to give one's favorite character what they want the most.
The book is an enjoyable read (not very long, if you're pressed for time), and I recommend it to anyone who is interested in science fiction, Arthur C. Clark's work, or looking deeper into the reference the designers are making with Krelian.