Your go-to works are Lavelle's Alfred's Wars and The Danes in Wessex.

Baker, Brookes and Reynolds' Landscapes of Defence and Baker & Brookes et al's Beyond the Burghal Hidage are important, as is Keynes' and Lapidge's translation of Asser's Vita Ælfredi. Higham's The Anglo-Saxon World is a good 101 for the period, but Molyneaux's The Foundation of the Kingdom of the English in the Tenth Century is an excellent in-depth look at how Alfred's reforms in the face of Danish invasion were leveraged into the machinery of the expanding English state. Strickland's Anglo-Norman Warfare may seem to postdate the period, but actually goes into some useful depth about the 10th and 11th Century English navy.