If you haven't read any Deryni novels before, then this isn't for you. There is probably too much background information which isn't included - having been gone into depth in previous novels, which, with all due respect, you need to have read.
This is a book about Alaric Morgan, a high-powered Deryni, whose loyalty lies with the Crown of Gwynedd: currently, in this book, with Brion; and in previous books, with Kelson, Brion's son.
It is a narrative of Alaric's life from age 7, to his 'majority' at 14, and details his training in courtly and knightly matters.
No, this is not a middle-grade read, for a number of reasons:
It is slow reading, and only needs perseverance.
It is full of page/squire/knightly oaths, and repetition can become boring.
There are a number of Moral questions posed, which with 'Deryni magic' could mislead.
There are also a number of gory descriptions of fighting and deaths.
It is interesting, for fans, to read about the early lives of various personages who have major roles in previous novels: for example: Oliver de Nore, Archbishop of Valoret and his immediate family; Denis Arilan, mentioned in passing by his brother Jamyl; and Duncan MacLean, who plays a major part with Alaric in the Kelson novels.
So, yes, brilliant for fans of Ms Kurtz; no, if you've never read any Deryni stories before.
And beware, Ms Kurtz does have a particular style of writing which may not appeal to all.