JPL DE Documentation
Be aware there are differing formats. SPK is the newer format - the older text format seems to be deprecated.

It's worth having a look at the documentation on this jplephem python library as it explains the format reasonably well. If you do use SPK, you'll need to read all of the documentation in NASA's SPK repository

I'm not sure if this is from the same author as this library - worth a read as it discusses the FITS format.