Digressions
Back in the day, Victorian novels were split into three volumes, but not for artistic reasons. It was simply the format that made sense at the time. Readers didn't have enough money to buy a full book, and lending libraries, like Mudie's and Smith's, wanted to have books split in three so that they could lend separate parts to different readers at the same time. It's a classic example of how commercial interest influenced artistic expression.
There's no more Mudie's in 2004, and books hadn't been split into volumes for a long time. Jonathan Strange & Mr Norrell still has three volumes (luckily, they are published as a single book, though). Volume I is Mr Norrell; Volume II is Jonathan Strange; and Volume III is John Uskglass. Susanna Clarke could have arguably split this into two or three novels in a series, another format that was very popular at the time. If you remember the Harry Potter craze at that same time, you can see that this would have been quite a viable decision, following commercial interest. It would also have meant readers waiting on it volume by volume, and I have written about what a wait like that can do to a reader. But the book is a pastiche, it resembles a Jane Austen piece in tone and format, and the more modern series format might not have been authentic enough. I also doubt that Clarke would have been happy to split the piece over an even longer time frame, considering that she spent ten years and considerable sweat and tears on this work.
The reason the three volumes stuck in my head is that some time ago, I read an essay about literary manspreading: male writers take up room as they like, they spread their digressions across pages and pages. And they get called ambitious for that. Infinite Jest, Ulysses, and Mason & Dixon. They've got long excursions into obscure subjects that a reader is expected to follow (but let's be honest, what percentage of readers really finish those books?). Most women writing at that scale would get called undisciplined.
Jonathan Strange & Mr Norrell has almost two hundred footnotes, some of them running for pages. It's an entire history of English magic, with biographies and folk tales and citations to fictional books. But the funny thing is, they don't read as digressive. They are part of the narrative, they build the world, they serve a function, but many of them could also be skipped. When encountering a page-long footnote, in Jonathan Strange & Mr Norrell, my heart jumps a bit and I wonder what mystical piece of magic I'll be able to explore next. A long note in Infinite Jest just makes me wince.
In all fairness, the plot could have been a little bit tighter. The first volume especially could have done with a bit more movement. But maybe that is also just part of the style. You simply cannot compare it to the contemporaneous books about English magic (such as the all-enveloping Potter novels released at the same time).
If you're going to read it, you should also read the footnotes, they are what makes it come alive. Skip them and you will only finish a long novel about two magicians who maybe like each other, or maybe don't. You will also miss the best part of the book, the one that took ten years to write and that is, arguably, the more interesting one.