I've read 4 of the 5 books in that Great and Terrible series. Actually, the series title is very apt, as it is both great and terrible.
As Ookla said, the series starts in the pre-earth life, taking what I consider to be reasonable artistic license in fleshing out the details of events that are only loosely described in scripture. It introduces the four brothers and one sister who will form the centerpiece of the rest of the series, as well as Lucifer and his chief lieutenant, who get several viewpoints as well throughout all the books.
Then, starting with book 2, the series portrays the conflicts and events leading up to the second coming of Christ. I assume that's where the series will end, but again, I haven't read book 5.
If you're LDS, the first three books are great reads. They cast the mortal conflict in a very real and tangible light, and the story and characters are entertaining. They mostly manage to avoid the cheesiness that plagues Mormon fiction. If you aren't Mormon, though, steer clear; you'll probably roll your eyes at every other page.
The fourth book was rather slow and felt like filler. The plot barely advanced at all, there was no real ending, and as an added bonus, a few of the scenes dialed the cheese factor to 11. Even still, I plan to read book 5 eventually, if only for closure (and out of hope that Stewart will repeat his prior successes rather than his failure in book 4).
Chris Stewart has written several non-LDS books. I've read The Fourth War, which is a military thriller. It was...decent. The writing lacked polish and it was poorly edited, but it still managed to entertain me. Stewart is a retired Air Force pilot, so he writes very believable soldiers and military conflicts, and his perspective on the wars we fight is interesting. That notwithstanding, The Fourth War isn't as well-written as The Great and Terrible, and reading it hasn't impelled me to seek out his other work.