There are a number of books in the overall series, and while I have only read the first 3, I am hopeful the rest have the same qualities:
1) Gritty action
2) Grounded Magic
3) Interesting characters
4) Difficult dramatic situations
5) No clean endings
I think the literature (regardless of genre) I enjoy most are those with characters who are complex and do not always do the right or even the best thing. I don;t like being able to guess where things are going or how things will play out within a story. I prefer "mud and blood" gritty details to high fantasy wispy scenes. I prefer lean prose to overly detailed descriptions. In all of these things Glenn Cook's Black Company excels.
In a nut shell the Black Company is a mercenary group with a long history and the tales are told from the Company's Historian point of view. The company (early on) ends up working for the bad guys (The Lady and The Taken) and there is a constant tension in the tales because of it. The Lady is an ancient evil who was wedded to The D0minator (think Sauron). While the great evil was defeated, its come back...bent on domination...and the Black Company ends up working for them. Imagine if the LotR Fellowship was actually working for Sauron but torn about doing it...that's about the most basic (although not accurate) sort of idea.
There are many more details and twists to the story arc, but I won't spoil it by telling here.
The books are short in keeping with their lean prose, and I've manged to blitz through them on the many plane trips for business I've had this year. I think they are quite good and if you are interested in a HOW military based campaign, or are looking for a unique take on villains and magic, this Trilogy is a great resource on top being a really good read.