Doctor Who, Sons of Thestian

Doctor Who 9.02: The Witch's Apprentice
( Spoilers )

The Sons of Thestian, by M.E. Vaughan
I read this because of a Tumblr rec & good goodreads reviews. That was a mistake. 90% of the book is boring and then it has a great ending, which is wasted because the buildup didn't work. Shame. ( Spoilers )