BOOKS 1-4 are about Jonas Faraday & Sarah Cruz.
BOOKS 5-7 are Josh Faraday & Kat Morgan.
These are all STAND ALONE BOOKS. They can be read in ANY ORDER, it doesn't really matter.
Newbies to these books are advised to start with HERO if you want to follow the time in which things happen. But again these books are STANDALONES and can be read in any order, just know you will jump around in time.