Hal knew that Dex hadn't read Nicks book....Gail hadn't read Nicks book
Hal had killed Nick and taken all of the copies from the flat except the one that was at Gails house.
Dex wouldn't have known it was Nicks book that Hal was passing off as his own, and just ask to borrow it from Gail telling her he would read it and try and give it a good grade as a memory to Nick....then just 'lose' it
Why kill them both??
Hal didn't know that Dex hadn't read Nick's book when he poisoned him. Dex tells him while he is drinking the poisoned liquor. He assumed that Dex had read it since it had been turned in long enough ago for Nick to ask Dex for feedback.
Hal goes to Gail's when he discovers her affair with Nick because he is concerned that she might have a copy. Ironically she has Dex's copy, which he accidentally left there and she has already begun reading, so he has to kill her as well.