Required reading to understand Grant Morrison's Batman
Here are a list of "suggestions" in the best order I can think of so you can see the hidden messages in Grant Morrison's Batman story. Some books you may have to reread more then once, since they have a lot of depth and other clues in other books are needed to understand what is happening. Understanding Grant Morrison's Batman is not going to be easy if you want to reach those deeper levels.
This list is here to give a guideline.