Well, if you enjoyed "The Book of Three," then I would rank "The High King" at essentially the same level of quality.

By this I mean that in terms of what is good, the characters are mostly entertaining. The narrative, however, is littered with a number cliche fantasy tropes. I would even argue that these platitudes work against the strength of the characters. This is most clear with Eilonwy, who had many interesting elements, but ended up being more plot device than character (again).

Was it amazing? Na-ah.
Did I enjoy it? More or less.
Do I recommend it? If you are completionist, then sure.

This is the highest praise I can give for "The High King."