CK-12 Foundation's Probability and Statistics (Advanced Placement) FlexBook introduces students to basic topics in statistics and probability but finishes with rigorous topics an advanced placement course requires.
CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the "FlexBook," CK-12 intends to pioneer the generation and distribution of high quality educational content that will serve both as core text as well as provide an adaptive environment for learning through the FlexBook Platform™.
Down sides: a) The numerous typos, particularly in equations and problem sets. b) Call me old school, but I don't really consider 'this problem is solved by pushing the following buttons on your calculator' to be much of a lesson.