The best source (of course) was Bishop Asser’s life of Alfred – a contemporary document, though there are historians who condemn it as a mediaeval forgery. I’m convinced it was the real deal and it’s an extraordinary book which tells us a lot about Alfred. It is hagiography, but there’s enough detail between the lines to suggest the truth behind the portrait. So it was Asser, plus all the books which I could lay my hands on – I’d recommend Justin Pollard’s book for a starter. We also have Alfred’s own writings – mostly translations, and the interest there is what he chose to translate – and his laws, of course, and the Anglo-Saxon Chronicle (which is annoyingly short of detail most of the time). We have an immense amount of material about Alfred (if only we had half as much for his successors). The picture that emerges is of an extraordinarily intelligent, diligent and pious man, obsessed with the church and education, who was forced into being a warrior king, though my own view is that his lifelong illness (probably Crohn’s disease) and his predilection for scholarship suggests that he was probably not the burly shield-warrior the statues depict!
How long do we have for this answer? I’ll be brief. Alfred lies at the taproot of English society and culture. Of course there were influential people before him, and kings who wanted to be the rulers of what became a united England, but Alfred defends the culture, language and religion at the point where it came nearest to extinction, and not only defended it, but in a sense codifies it. He’s a great man for writing things down, thus the birth of the Anglo-Saxon chronicle, his own laws, his translations. He’s intent on education, so that the ruling class of Saxon Wessex (and Mercia) understand his aims. He never achieved the primary aim, which was to unite the English-speaking peoples of Britain, but his successors did, and they were following a blueprint laid down by Alfred. He also institutes good government, which survives all the turmoil of the next four centuries. There were great kings before him, but it’s hard, for instance, to say what legacy Offa of Mercia left (they now even doubt that he built the famous Offa’s Dyke), but Alfred’s legacy still lives on.
I think it’s a pity that the English, on the whole, are ignorant of the Saxon period, and have no idea where their nation came from or how it was created, but on the other hand there has been so much history since that it’s surely understandable that the Saxons get buried beneath all that comes after them. They’re not forgotten – and even Hollywood nodded in Beowulf’s direction.