It took Susanna Clarke 10 years to write this, her first novel; it's exceptional
not just for weighing in at about 800 pages (an extraordinary thing for a first
novel) but also for the full realized world that she creates. Many
reviewers have described it as Harry Potter for adults. To the extent that
it's a book about magic set in a world like, but not quite like, our own, it is;
however, like most comparisons, you can only take it so far.
Interesting link: A complete short story: "The Duke of Wellington Misplaces His Horse", part of her 2006 collection, The Ladies of Grace Adieu.