I recently bought a Kindle and I have splashed out on a lot of titles I normally wouldn’t have bought. My overall expenditure on reading has increased, in the short term at least. I still buy printed books and I still borrow titles from my local library. For holiday reading I plan to use my Kindle whereas last year I would have bought a couple of paperbacks so there is a slight decrease in my spending on traditional books.
I agree with Stephen Fry that electronic books can exist side by side with print. It is however very easy to order printed and electronic copies online and this is definitely a challenge to bookshops.