I've recently written a comedy travel book, and the following were all excellent inspirations, even though some of them aren't really travel books.
In Search of Captain Zero - Allan Weisbecker. Not as funny as Cosmic Banditos, but more based on reality. More of a classic travelogue.
Solomon Time - Will Randall. English teacher moves to the Solomon isles. Charming English humour.
Los Angeles Without a Map - Richard Rayner. More about the craziness of LA than travel, but has plenty of laugh out loud funny moments coming from someone being outside their normal environment or comfort zone.
You Shall Know Our Velocity - Dave Eggers. Too many themes in this book to try to sum it up succinctly, but there is some beautiful travel writing in there.
Off The map - Hib and Kika. Unconventional travelogue of two American hippie women in Europe. I hate to use the word charming twice in one post, but this is exactly that
Three Men in a Boat: to Say Nothing of the Dog - Jerome K. Jerome. Old classic but one that is still