I read the 3 first books in this series (The Last Kingdom, Pale Horseman, and Lords of the North) quite some time ago, found them all three excellent historical novels, so started "Sword Song" with high expectations. Rest assured: it's near perfect.
In "Sword Song" the struggle between Saxons and Danes has reached a new phase, with Alfred consolidating the kingdom of Wessex and trying to get a grip on the (leaderless) Mercia. Uthred has become Alfred's most important warlord, but he feels equally drawn to the Viking-world in which he grew up, and just then an army of Vikings arrives and captures London...
I really can't do this book enough justice: the battle scenes are as good as ever (and there's plenty), but above all I love the way in which Uthred keeps developing as a character. The arrogant, young warrior has become a mature man (with a fearsome reputation, true enough), and a loving husband and father.
I for one am eagerly awaiting the next book in the series.