Pournelle,always an excllent sci fi writer (the masterful "Falkenbers Legion, for example) teams up with another military pro, S.M. Stirling, to continue to create this ravishing tale, a sort of prologue to the seminal "Mote in Gods Eye"
The style of the piece is typically down and dirty. The characters aren't special people, or heroes. They're just people for whom fighting and killing is an occupation - they do what they have to, which happens to be heroical. Some of the mercenary edge has come off the prose dince "Go Tell the Spartans" but this is perfectly in line with character development from the previous book.
One flaw is that while the text can be read a sa stand alone, its hardly advisable; the back story from "Go tell the Spartans" is almost necessary, not to mention the prevous two books in the series. However, once you get past that flaw (hopefully by reading the other books) this is an excellent addition to the genre. Thecharacters are complex, well motivated and can be identified with - their essential humanity is visible, whichever side of the war they are on. Indeed, both sides all too often appear to be in the grey area, rather than "Good" or "bad" though the reader is led to be more sympathetic to one side. The military setpieces are incredible and will leave the reader turning pages fast enough to get paper cuts, which makes the overall work extremely good.
Buy it. But buy the other three in the series first. Then buy it. Its really, really, really good.