Mine is "Europe on 84 cents a day" by Gil White.
This guy trek'd Europe back in the 70's before it everyone caught on what an amazing place it was, before the rush of tour groups crowded the streets and cafes. I don't even know if this book is still in print, I got it back in 1995...In the 70's this guy traveled for 84cents a day...That would be amazing!!! Now it is much more $$$ to do, but if you are dedicated, you can apply all the same principles. I refer to it often, and enjoy reading it more than any other book I have.