I'm currently reading "The Jane Austen Handbook" by Margaret C. Sullivan. It's a beautifully produced little package--handsome paper, charming fonts and borders, and delightful illustrations. If you are new to Jane Austen's books and the Regency world, then this book is for you. It covers all the aspects of that society that will be new to you, and it will introduce you to this new world in a light-hearted, tongue in cheek manner.
I'm a little disappointed in the content however. Anyone who has read Jane Austen, Georgette Heyer and/or a host of others will not find anything new in this Handbook. Still, all the information is laid out in an easy to find, easy to read manner. There are several useful appendices; a glossary should you still have any questions about Regency words, a short bibliography and other resources.
All in all, the Handbook is a charming addition to a Jane Austen bookshelf, though not a must-have reference book.