Take a look at the thread here titled "Latest and Greatest about Delta" or words to that effect - it's a huge thread but wade through it for the interview questions and study guides. Also look at Will Fly for Food and aviationinterviews.com. There are a couple of other threads here that have more interview specific questions for DAL.

You might also look at some of the books recommended such as Checklist for Success (<60 minute read) by Cheryl Cage.

Hope this helps and good luck!