Anyway, back to the headline question. The longest single-word palindrome is, sadly, not in English, but in Finnish. The word is saippuakivikauppias and it means ... soap-stone vendor. This raises the question, "Why did they need a word for that?" (Actually, I'm guessing Finnish must be like German, which pushes multiple words words together, as in the German herzkreislaufwiederbelebung, which literally means "heart-circle-run-again-revive," or as we say in English, "C.P.R.")
Gardner also included examples of "whole-word" palindromes, in which a sentence can be read forward or backward word-by-word. For example: You can cage a swallow, can't you, but you can't swallow a cage, can you?