Dreaming of diaries means you are going to check your old entries for information. It is also a reminder for your brain to be motivated to record down information regularly on a daily basis. Reading old diaries has a different meaning. One is reading the past for lessons learned, to prepare for the future. This is a warning on a threat.
Writers and artists keep journals and diaries to record ideas. If you are one of these, then dreaming of diaries may hold another meaning for you. The message may be for you to search for a particular entry with a specific topic. Or start working on your old ideas soon.