Start with Cambridge’s own ASNC introductory reading list, because it anchors preparation in the course’s actual fields. Higham and Martin J. Ryan, and Early Medieval Britain, c. 500–1000 by Rory Naismith as useful starting points.
For the Norse side, The Viking World and Chronicles of the Vikings give you ways into Viking Age evidence and source-led discussion. For the Celtic side, Wales and the Britons, 350–1064 is listed as useful preparation for early medieval Britain beyond England.
Language tasters help because ASNC includes Old English, Old Norse, Medieval Welsh, Medieval Irish, Medieval Latin and Palaeography among first-year options. Old English Online is a beginner route into Old English, while Icelandic Online supports early interest in Old Norse and Icelandic saga culture.
For listening and video, Saga Thing is useful for saga culture, In Our Time models evidence-led academic conversation, and the British Library channel is useful for manuscripts and medieval collections.