Having read the notes in addition to the novel, it is clear that Jonathan Strange and Mr Norrell (by Susanna Clarke) is one of the most real fictional subcreations I have ever encountered.
It is very difficult not to believe it really happened, because the world is so detailed, so consistent, so convincing. As I walked around Newcastle today, I kept thinking of the time when the Raven King ruled from here - I even saw a raven!
I was trying to the think of comparisons in the post-Tolkien literature.
The nearest fantasy I could come are the Alan Garner Weirdstone of Brisingamen/ Moon of Gomrath duo in which the magical events are sewn-into a lot of local and family history, folklore and neo-paganism. The ring of truth and believable - but the detail is much less than in JS&MN.
The other example is the Glass Family stories by JS Salinger, which are also difficult not to believe, and similar in their detailed and deep and partly factual back-story (albeit in a different style) to JS&MN.
('Difficult not to believe', that is, with the exception of the last Glass story published - Hapworth 16 1924, which is simply atrocious).