The Eyre Affair
By Jasper Fforde
Penguin Books, 2001
Well, really, Lord NAgasaki's review is right on target. This is a great book, and you should read it. However, I may be able to add a bit more information on the feel of the book, although Jasper Fforde doesn't make that easy.
The Eyre Affair is most often found in General Fiction, although it would be fair to put it in Science Fiction. My local library has it in the Mystery section, which is pushing it. If you really wanted to be exact, you would probably say that it is a comic science fiction/fantasy crime novel. Whatever you call it, it's an excellent read.
The sticking point for most readers, I suspect, will be the 'comic' label. This is very much a silly book, and at times it is too silly even for me (and I am a long time Douglas Adams and Terry Pratchett fan). The alternate reality that Thursday Next and friends live in is often a lot like ours, and usually within the envelope of magical realism -- that is, most characters certainly believe that they are living in a mundane, everyday world, but some characters happen to live in a part of reality that intersects with vampires. Or evil supervillians. Or time travel. But some of these bits of science fiction/fantasy fall outside of credulity, even if the audience is perfectly used to reading books about vampires and magicians.
The supervillain (well, the biggest supervillain), Acheron Hades, is the sort that fits better in a pulp comic book than a modern novel. The idea that you can travel into and out of books doesn't sit well with the comparatively sane storyline of a police investigation. And the mad inventor invents very silly things indeed, such as a device to get energy from meringue.
The upside to this is, the book works, and works well -- so you know it has to be really great, to overcome the silliness so completely. It has a well-written plot, likable characters, and lots of surprises. A lot of the odd bits fit very well into Thursday's world, making the story more cohesive and interesting, and the too-silly bits are treated with a matter-of-fact attitude that keeps them from being overwhelming.
It is also worth noting that Eyre is written for a fairly intelligent audience, and even more so, for a well-read audience. There are constant literary references, in part due to the plot, but mostly due to the world. The characters go to plays in place of movie theaters, small talk as often as not means discussing Shakespeare conspiracy theories, and literary landmarks are big tourism. And this works really well -- and moreover, it works really well even if you hate small talk and don't care who wrote Shakespeare.
As mentioned above, this is a not a stand-alone work. The series thus far consists of:
In my opinion, The Eyre Affair is by far the best of these, and while the later books in the series are worth reading, you should also consider reading some of Fforde's other books, particularly The Big Over Easy and Shades of Grey. But by all means, start with The Eyre Affair -- you won't regret it.