The Joseph Delaney The Last Apprentice series is extremely popular in my school. The Spook's Tale and Other Horrors will not sit for two minutes on my desk before some eager child checks it out.
This tells three different stories-- Tom Gregory's beginnings as a spook,
more about Alice the witch's past, and Grimalkin's account of how she became a witch assassin. There is also an overview of the other creatures that appear in the series. This would be a good introduction to the series, or appeasement for those who have read book five and need something to hold them until the August 25th release of Clash of Demons.
I do love these, even though they involve a lot of evil demons and horrific situations. Upon reflection, I think that what is best is the conflict that the characters feel. Sure, the monsters are pure evil, but the witches do have some humanity. The story of how the spook ended up as an apprentice instead of a priest was a prime example of the philosophical choices that many of the characters face. The spooky illustrations and the whole presentation of the books is wonderful as well.