Keegan has written some of the best military history in the last 30 years, IMO. He is well worth the time it takes to read him.
My favorites by Keegan:
A History of Warfare
Face of Battle
Mask of Command
Price of Admiralty
There are two areas where Keegan stirs controversy.
1. Carl von Clausewitz
Clausewitz aficionadoes (Clausewitzians?, Clausewitzites?) foam at the mouth at the mention of Keegan, because Keegan takes some pot shots at CvC. I'll admit Keegan sets up a bit of a straw man to whack down, but CvC gave hime some of the straw to work with...
Also, I think the CvC'ers are jelous that Keegan writes a book that is well-written and easily grasped by educated non-historians. CvC has the 18th/19th German authors' tendency to write dense, nearly impenetrable prose (by the layman & those not steeped in CvC's time period) that tries to fit the facts to his system...and used a heavier hammer when they do not fit.
2. Keegan's PC-ish Book Conclusions
JK has a tendency to write a wonderful book and then tack on a conclusion that sounds like it was written by a different man or one that is at odds with the material presented. I thought about it for a while and have two opinions about it.:
A. JK is English, grew up during & after WWII, was one of the last kids to get polio beofre the vaccine, and is a product of the dissolution of the Brit empire. His take seems to be one of, "Well, we can't do that sort of thing anymore. We just don't have what it takes."
B. JK tacks on the conclusion as a sop to the PC/anti-colonial/anti-western culture academics who are his colleagues.