Deepest Night starts not long after Sweetest Dark ends. I do think this series should be read in order. There is more emphasis on the ongoing world war this time because of Mandy/Armand’s brother Aubrey, who is a soldier and behind enemy lines. Eleanor is still dealing with issues from the last novel and not interested in a more romantic relationship with Mandy, even though he desires it. Eleanor’s ability to become a corporeal dragon/drakon becomes a major apart of the plot line in this novel, and that drives the storyline more than the romance aspect.
I’ve ordered both books in this series for my high school.