Your go-to primary sources are The Anglo-Saxon Chronicle and Asser's Vita Ælfredi. Exercise standard cautions. It's also worth looking through William of Malmesbury.

Your standard work is Stenton's perennial Anglo-Saxon England. There's a whole library of literature on the reigns of Alfred and his descendants, but I'd briefly recommend Ryan Lavelle's The Danes in Wessex and Alfred's Wars, Hill et al's The Burghal Hidage, Baker and Brookes' Beyond the Burghal Hidage, and Baker, Brookes and Reynolds' Landscapes of Defence.