Ten years in the writing, Jonathan Strange & Mr Norrell by Susanna Clarke is a masterpiece. It is one of the finest fantasies ever written. It is one of the best books I have ever read. My copy is 1006 pages long, and I enjoyed reading every single one of them. If you are a regular reader of fantastic literature and you have not yet read this book, I strongly recommend that you do so.
For someone who is not a regular reader of fantasy, the setting is mostly an alternate England during and shortly after the Napoleonic Wars. The two most important characters are the gentlemen in the title, two magicians with the goal of bringing magic back to England. Their characterization is contrasting, well rounded, and complex. All of the important characters are well rounded and believable. There are no cardboard cutouts. There is no melodrama.
The dialog is often witty, sometimes in a dramatically ironic way. The book contains an immense amount of dry--positively desiccated--humor. Those susceptible to it will find the humor alone makes the book worth reading. As I read, I often found myself grinning and chuckling aloud.
Ms. Carke's writing is clear and clean. For this novel, she chose to use the omniscient viewpoint, and the narrator occasionally addresses the reader directly. It's an old-fashioned technique decried in modern writing manuals, but the author uses it skillfully, conveying much of the humor through observations in the narration. Personally, I didn't find it obtrusive, and it didn't interfere with my reader's trance.
Readers who might want to avoid the novel: As stated above, it is one fat book. Those who read to get to the end will have a long journey. Despite the length of the book, it does not have a large number of superfluous side plots, but still, the novel sprawls a bit. Those who like tightly plotted, straightforward action should probably give the novel a pass. Obviously, those who believe fantasy is puerile shouldn't bother reading it.
Fantasy readers who stick to gaming-based worlds or paranormal romance, and who are uninterested in expanding their horizons, probably would not like Jonathan Strange & Mr Norrell. On the other hand, kids who grew up with Harry Potter but who are now adults might want to give it a try. (I think it's a whole lot better.) For serious fantasy readers who have not yet read it, I can tell you that I think Ms. Clarke's creation might be an even grander achievement than Neil Gaiman's American Gods. That's how good I believe it is.