The great thing about the web is that you can read a book and turn to a huge online resource of interesting article, free translations and extra stuff like photography of Prague in the time when Franz kafka lived there. A few years ago - pre children when money was more available - I visited Prague and picked up a couple of Kafka's Prague type books and the great thing is that most of that material is now online for free. If you have never visited Prague then it is a great place, not just for Kafka lovers, and if you get the chance go.
A good starting point for Kafka links is the Franz Kafka on the web page that links through to sites in various languages (although be warned some were not working). Then there is of course the obligitary Wikipedia reference. But one that stood out was The Ledge, which provides summaries of the books and suggestions of further reading but has a flash interface that is really worth having a look at and has lots of interviews ana summaries of other books.