I’ve had a subscription to Safari for over five years now. For a monthly fee (pricing is dependent on the plan you choose), Safari grants you instant access to thousands of tech and business-related digital books. New titles become available surprisingly regularly and quickly (occasionally Safari will get the digital version of a title before Amazon does). In short, the service gives me access to a wealth of knowledge in a much less expensive and more convenient manner than any alternative.
[Here is a list of titles that Safari offers as part of their service.--OH]