Thursday Next is an all grown up Nancy Drew if she were written by Douglas Adams in an alternate reality where the most horrific crimes include kidnapping and ransoming characters from classic literature. The book itself is not my favorite in the series; I think the series gets progressively better with each book. This particular novel spends a good deal of time setting up the world around Thursday Next and the different alternate realities in which she can and will travel to in the future.
 
The Eyre Affair is the first book of the series. I read it about ten years ago. At the time, I had never read Jane Eyre. The man who recommended it to me told me that it didn't matter, and he was more or less right. I still enjoyed it immensely and went on to read the rest of the series in quick succession and it overtook the top of my favorites list immediately.

I'm really not a fan of rereading books under normal circumstances. My to-read list is WAY too long to spend time reading something I have already read. However, since I just recently read Jane Eyre, and I am finally hearing rumors about the eighth book in the series coming out soon, it felt right to give this a second run.
 
All that said, this book is way better after having read Jane Eyre! While I agree that it isn't *necessary* to have read it (the author gives you a synopsis within the story so you at least get the gist of it), it was much more enjoyable to read it with formulated opinions on the characters of Jane Eyre.