I really enjoy the well-researched insights that Keegan brings to life in his books. Battle at Sea deals with the development of naval warfare from the wooden battleships of the gunpowder age to the nuclear subs of the present. Keegan shows how war at sea has evolved in the last few centuries and gives his assessment (or, let's be honest - qualified guess) of how naval warfare will evolve in the near future.
The book is well-written - though the copy I received had a remarkable amount of spelling-mistakes; poor proof-reading? - and it offers a very clear picture of the times it portrays. The battles seem clear and straight-forward even-though massive amounts of ships, sailors, planes and submarines are involved in them - overwhelmingly confusing to the participants, no doubt, but clear to the reader, thanks to Keegan's guiding hand.
This book, however, doesn't quite reach the level of brilliance that Keegan shows in Face of Battle, Mask of Command and in his two epic volumes about the First and Second World Wars; leaving this a very good book, but not a masterpiece.
In spite of this, it is still highly recommendable to anyone who has an interest in naval warfare or in military history in general.