Theoretically you can find everything on the Internet but getting it all in a single volume for a moderate price is so much easier. This is the most complete guide for a gay tourist - not only gives you details of your probable favourite spots in places you have never visited before but offers a few words of warning and introduction.
Still one comment is necessary - the more info you can find here about local attitudes towards gay men and anti-gay laws (in European terms: the further east you plan to venture), the less likely it is that addresses of clubs are still valid. Some Internet search before leaving home may be quite advisable if you don't want to find a grocery store in a place advertised as a gay watering hole ;)