Oh my, where's this world heading ...........
Tomas - firstly: thanks.
Secondly - I had the same problem too at first (at normal mode though). The book would heal up way more than I could ever harm it.
But this finally was the solution for me (and then it went quite smoothly):
Evil Scimitar on Hamilton plus the Cyber Glove to give him a second action.
Ordeal Sword and Assault Gauntlet on Priscilla (because the latter adds Evil to the attack). That way she does deal nice damage.
First round: Hamilton does Power Strike twice to boost his attack.
Then Hamilton and Priscilla attack, attack, attack (when you're lucky it's gonna be critical several times).
With the others I healed up or tried some spells, but Hamilton's and Priscilla's attack was what finally killed that book.