The other is more regularly updated, and produced by fans. Treasures found therein include a teacher's guide to Sutcliff's novels, and another article on the novels linked by the Dolphin ring (in three parts: I, II and III).
I learnt a lot of stuff I hadn't known in these pieces, such as the fact that The Shield Ring, in terms of internal chronology the last in this sequence, set in the Lake District in the time of Henry I (and which is slightly mixed up in my mind with the Norman novels of Jean Plaidy, which I read at about the same time), was actually written after The Eagle of the Ninth, but before any of the others.
I was trying to figure out why I hadn't read either Frontier Wolf or Sword Song, both of which are connected with the sequence. I haven't read all her works by any means, but I did make an effort to track down all of those with the Dolphin ring in them. But as I recall now, I was doing this in 1978-79, and these two novels hadn't yet been written.
There is one other novel in the sequence I haven't read: Sword At Sunset, her historical Arthur novel, and the only book she wrote for adults rather than for children. I did try. But, though Sutcliff said there was no difference between her writing for children and for adults, that's not how it felt to me. I found the prose in Sword at Sunset turgid and impenetrable.
Final note: the IMDB entry for the BBC TV version of Eagle of the Ninth makes no mention of Sutcliff. Naughty.