As far as I know, simply written records McCabe kept in his duty as the No. 2 at the FBI, perhaps recording interactions with other members of FBI/DOJ or perhaps with the POTUS, or perhaps dealing with ongoing FBI matters related to FISA and Trump/Russia investigation and some such. I don't know if any of their specific content is out there now, although there is a claim that the Rosenstein wearing a wire and 25th amendment claim, was a story leaked from McCabes personal FBI memos.