Gatekeeper of Apollo

The Gatekeeper of Apollo is a book by Nicholas Goubert.

Jonathan Strange is reading a copy in Mr Norrell's library when he is suddenly transported to Mr Norrell's location by the Hurtfew river, where he had gone to fetch some water for a spell to locate the Raven King. The two magician surmise that not only are they trapped in the Eternal Darkness together, but they can also not move very far from one another [66].

During his conversation with his wife, Jonathan Strange mention the book as a source he and Norrell are using in their search to find a means of breaking the spell which keeps them imprisoned. Strange tells his wife that the book has "one or two things" which "look promising"[69].