Here is the beginning of the list of John Piper books that are available for free in various formats (but mostly pdf):

• 50 Crucial Questions about Manhood and Womanhood (pdf)
• Adoniram Judson (pdf, epub, mobi)
• Amazing Grace in the Life of William Wilberforce (pdf)
• Andrew Fuller (pdf, epub, mobi)
• Beyond the Bounds (pdf): Open theism
• Bloodlines (pdf): Racism
• Contending for Our All (pdf): Brief biographies of Athanasius, Owen and Machen
• Counted Righteous in Christ (pdf) Double imputation in justification
• David Brainard (pdf, epub, mobi)
• Desiring God (pdf)
• Disability and the Sovereign Goodness of God (pdf, epub, mobi)
• Don’t Waste Your Cancer (pdf)
• Don’t Waste Your Life (pdf)
• Exposing the Work of Abortion (pdf)
• 50 Reasons Jesus Came to Die (pdf)
• Seeing Beauty and Saying Beautifully (pdf)