Those of you who are familiar with The Importance of Being Earnest by Oscar Wilde (either the play or the movie) may recall much being said about the three-volume novel, which was spoken of rather disparagingly by most of the characters in the play.
Back in Austen's time, lots and lots of models were three-volume novels. Not because a novel couldn't be printed all in one volume, but because of the circulating libraries that existed at that time. Circulating libraries were very popular, because books were (relatively) expensive, and only a small percentage of the populace could afford to buy as many as they liked. (This is one of the reasons that wealthy people in novels set in that time, whether written then or now, tend to have large libraries - it's a form of conspicuous consumption, really, even if one has to be inside the walls of their homes to discover that it exists.)
Circulating libraries required a paid membership subscription, and one usually paid a fee in order to borrow a book. That's right - the fee was paid on a per-volume basis, not on a per-title basis. A novel printed in three volumes was therefore worth three rental fees, rather than just one. Publishers knew that most novels would sell a significant percentage of most novels to circulating libraries, so they were only too happy to accommodate those libraries by printing the majority of their novels in multiple volumes.
And thus it was that Sense and Sensibility, Pride and Prejudice, Mansfield Park and Emma were printed as three-volume novels. (The other two novels, Persuasion and Northanger Abbey were printed together after Austen's death in a four-volume set.)
Don't forget our foray into Sense and Sensibility starts this Thursday. If you haven't found a copy of the novel, but would like to read along anyhow, may I recommend Molland's Circulating Library, an online repository of Austen-related texts? Here's the direct link to the Table of Contents for Sense and Sensiblity.