Professor Deborah Geier has published the updated 2017 edition of her freely downloadable open textbook: The Federal Income Taxation of Individuals 2017. In addition to updating the textbook for changes in the law, for the first time, Professor Geier added hyperlinks to significant Code sections and Treasury Regulations as they are discussed in the text. She cites to the Legal Information Institute at Cornell University, an open-access resource. The textbook is available in Word, PDF, iPub (for iPads) and MOBI (for Kindles). Here is the link: https://www.cali.org/books/us-federal-income-taxation-individuals.