Just finished "Lords of the Sith", and I must say it ties with "Lost Stars" as my favorite (thus far) of the new Disney Star Wars canon. It definitely continues in a trend of more brutal and "realistic" war stories, shoved into the Star Wars universe. In my view, that is both a good and bad thing. On the good side, the stories seem to be more serious and visceral. On the bad side, there are lots of mentions of head shots, entry wounds, gore splatter, and various other graphic and extraneous episodes of violence.
I won't give a lot away, plot-wise, but this book takes place almost entirely on Ryloth, and includes Cham Syndulla (Rebels Hera's father, himself a Clone Wars character). The action is very fast paced and exciting, the scope is perfect for an early Empire work (i.e. don't overshadow the Death Star or anything). Just seeing Vader and Palpatine in action, side by side, is worth the price of admission.
As a side note, I recall some chatter regarding a gay character in Aftermath (which I have yet to read). In this book, the female Moff in charge of Ryloth is said to have had a wife who died some time ago in an accident. Another nod to the Disney Star Wars being much more inclusive, overall. I thoroughly approve.
All told, "Lords of the Sith" is a very nice addition to the Star Wars canon.