Esperanto is explicitly mentioned by the fabulously famous novelist Graham Greene in his novel “The Power and the Glory”. The relevant passage is as follows:

It was an English book, but from his years in an American seminary he retained enough English to read it, with a little difficulty.
...
It was a very obscure poem, full of words which were like Esperanto. He thought: So this is English poetry: how odd.

Tre bone! Mi ŝatas leĝi Graham Greene!