There are four good advanced books out right now, and three or four bad books that have advanced in the title. These good ones are Deeping Witchcraft, Advanced Wicca, Wicca 333 and Elements of Ritual.
Of these four, it is difficult to say which is best, because each of them provides a different set of advanced information.
Of them, Deepening Witchcraft is probably the most intermediate book... the one for the student just ascending to the 200 level.
It contains much more than any 101 book, and makes assumptions about what you know, but people WILL be disappointed by the lack of "higher" advanced material in the book.
I, myself, am probably above the target age and level for this book, but my 18 year old daughter, Wiccan for 2 years, loved it, inhaled it even.
If possible, glance over it before you buy it and judge if it's below your perceived level of studies... that way, you won't be disappointed and complain that it's not advanced.
It fills a niche, though, that's for sure.