EE Ltd stands for Extreme Escapes Limited. It's the private contracting firm (think: mercenaries) owned by Phil Butler, and it's where Prophet and Tommy from the Hell or High Water series work.
The following books are set in the EE Ltd universe:
The Hell or High Water series books ARE part of the EE Ltd, universe. However, I'm limiting books labeled Hell or High Water to those books that are only about Prophet and Tommy. Therefore, the books in the EE Ltd universe would most definitely add to your enjoyment and scope of the world, but aren’t necessary to read before you read the Hell or High Water series.