This from Tim O'Reilly via Tim Bray ongoing - The Java + Open Source Sweet Spot. When books were the primary vehicle for knowledge transfer I used to measure success/adoption by shelf inches at Stacy's (Great Technical bookstore in San Francisco). Tim O'Reilly now has a more scientific approach - and please increase the graph size, I need a magnifying glass to read the data.
However I am not sure this now tells the full story. I have noticed a substantial drop in my book buying habit, much to the relief of my wife who has had to navigate the piles around the house. It is not that I read less but rather the internet is more the source of up to date information rather than a book. I have mixed feelings about this, as I enjoy reading information from a book, especially sitting out on a warm sunny day. Laptops just do not cut it for reading outside or on the train etc. While the information junkie in me loves the immediacy of the internet and ability to research any problem quickly and easily. Hopefully tablet machines will get to the point where they are readable in all environments.