It is not surprising that it took Susanna Clarke 10 years to write this book. It is phenomenal; one of the richest and most engrossing books I've ever read. The style perfectly mirrors the setting, and the multiple overlapping plot lines dovetail beautifully- I was shocked when I realized it was a debut novel. I've read it twice so far and plan on reading it again. Even the footnotes are wonderful.
In 2004, it won the Hugo Award for Best Novel, the Nebula for Best Novel, the Locus Award for Best First Novel, the Guardian First Book Award, and was longlisted for the Man Booker Prize. Not bad for a first time.
(There is a BBC mini-series coming out...sometime. The bastards haven't said when yet. I know it will never happen, but if they could get Benedict Cumberbatch to play Jonathan Strange, it would be the BEST THING EVAR.)