A GOAT novel written by Paul Cornell, one of the 4 or 5 writers guaranteed to always write a good Doctor Who novel. It's part of a quadrology, and the reason why I didn't make an entry for the other 3 books is because they're considered to be crap. (For example, Ace gets molested by Gilgamesh. Not as fun as it sounds.)
It stars Seven and Ace battling against...well, what may or may not be an ancient goddess of evil. What I can tell you is that the book includes 7 walking aimlessly through his own mind while also being dead in a sentient, talking church, Ace getting involved in a bullying PSA in which she dies at least once, and 5 gets crucified on a tree. The plots ends up being resolved by 7 doing self-therapy. It's a confusing book (rating at least a 8.5 on the Moffat Mindscrew Scale) but also GOAT. Read it!