Time to read some more.

I've read tales from the empire, tarkin, Death Star, darth Pelagius, and the new lords of the sith

What other good imperial Star Wars books are there?

The Darth Bane series of books are a pretty good read as it lays down the foundation for the Rule of Two for the Sith. I believe the order is Path of Destruction > Rule of Two > Dynasty of Evil.

There aren't too many of those, but The Thrawn Trilogy, about Thrawn and Pelleaon, and Choices of One, about Mara Jade.

...Choices of One, about Mara Jade.

Allegiance is the Zahn novel that precedes Choices of One, and introduces many of the characters.