My favorite part of "Half Magic" was the visit to Arthurian Britain. I thought that Eager particularly handled well the explanation for why Camelot fell, without directly bringing up the love affair (which would be tricky for a children's book - although if you read the tournament scene closely, there are hints about how Lancelot and Guinevere feel about each other); Agravain, Mordred, and their friends were fed up with Lancelot always unhorsing them in the jousts and figured that if they could have him disgraced, they might be able to win the prize for a change (and certainly get some revenge). I do get the impression from Malory that that was why they were so keen on exposing Lancelot's affair with the queen.
I wonder if anyone here's read one of the sequels, "Knight's Castle", in which the descendants of the MCs in "Half Magic" encounter Ivanhoe, Richard the Lion-hearted, and Robin Hood (the other classic medieval British legend).