This Quantum World
This is a free, online textbook offered by WikiBooks. As such, it is continually being updated. According to the authors, "In quantum mechanics, states are probability algorithms. We use them to calculate the probabilities of the possible outcomes of measurements on the basis of actual measurement outcomes. A quantum state takes as its input one or several measurement outcomes, a measurement M, the time of M, and it yields as its output the probabilities of the possible outcomes of M."