Delta sends interviewees a 7-page study guide showing the topics that will be covered on the Job Knowledge Test. In the email they send inviting you to interview, there is a link to download it.

The questions do not come from the FAA ATP test bank, they are from their own internal bank of questions. Lots of gouge available online about the test questions, but that gouge recently became outdated when the whole test bank was changed (about the beginning of February). Intel on the new questions say they are similar to the old ones, and that old gouge continues to be a good guide for studying even if the questions themselves are all different.