I'll begin by saying that I have only read the Kim Harrison contribution to this anthology, as part of my reread of the whole series in the lead up to the new release "White Witch Black Curse". So this review is only about the Kim Harrison short story, "Two Ghosts for Sister Rachel".
Without giving any spoilers what I will say is that this is the most interesting of the Hollows short stories, and shows us an 18 year old Rachel before she has joined the IS. This story is a mini-adventure that gives Hollows fans a little more insight into Rachel's background which I am certain will be important in future books (and probably WWBC). Most notably this story has a cameo apperance from Rachel's brother Robbie, who up until this point has only been mentioned once in passing.
For people new to the Hollows series this short story will mean very little. For dedicated fans this is a must read, and at about 100 pages it's a decent enough length to justify the purchase of the whole book. Characteristically well written, this is a good addition to the series.