One of the amazing gifts of the internet community is that the books we would buy (sometimes spur of the moment) are now often available online for free in pdf form. This is a form of stewardship that I think has some positive implications for the shrinking down of extraneous books in my library. Books that often sit there and are rarely if ever read by anyone will now never find my shelf as I have so many free options to read online. (it is a bit harder to read on the computer- I recognize that)
In this regard, Desiring God Ministries has done us a great service in putting many of John Piper's teachings free for us online - including some very good books. His passion for God-centered, Christ exalting lives of learning and worship is a constant source of contagious joy for my ebbing spirit. One of his new books is called What Jesus Demands of the World (see link below). Let me know what you think of it as I am currently studying it with a friend.
http://www.desiringgod.org/ResourceLibrary/OnlineBooks/ByTitle/

GF