Information about the reading order of Zero Hour comics can be found online here. The history and a summary of the Zero Hour storyline is found on the DC Database.
Zero Hour, 5 issue series
Zero Hour, issue 2, Sept 1994
Zero Hour "Zero Month" comics numbered as 0
Batman, issue 0, Oct 1994
Superboy, issue 0, Oct 1994
Zero Hour Tie-in issues
Detective Comics, issue 678, Sept 1994
Batman, issue 511, Sept 1994
Shadow of the Bat, issue 31, Sept 1994
Robin, issue 10 and Superboy issue 8, 1994.
I like how these covers mimic each other showing the original hero with the newer version.
