I just finished reading The Masque of Vyle. I found it a good read and a character from Path of the Incubus appears in it. The book gives more detail on the Harlequins and their setup, and how they execute their performances.
I found the plot interesting, but over fairly fast. Not a terribly long book, being between 1/3 to 1/2 of a normal warhammer book, which likely is why they made it as a hardcover too. Black Library seems to love hardbacking their shorties.
I rate it 7.5/10, a decent read with interesting spots. But being too short to truly delve deeply into most of the characters in the book. One could consider it a sidestory to Path of the Incubus, given the relation to one of the important characters there and detailing him with a suprise at the end.