You'll understand, when you read the rest of the book, why Uhtred calls himself an earl which is, as you rightly say, a Danish rank and did not enter English usage until Cnut - but there is a reason!! Ealdormen? Some were appointed by the king, but many families were so powerful (like the real-life Uhtred family) that they arrogated the rank and no king was strong enough to take it away. There certainly are hereditary ealdormen - lots of them - though doubtless the kings would have preferred it otherwise.