for one of the possible starters, I would go for the MIT Licensed "Paradigms of Artificial Intelligence Programming" by Peter Norvig. https://github.com/norvig/paip-lisp

a) you get the extra benefit of playing with Lisp b) it gets universal praise c) as you guessed, it's free in the freedom sense

There are other resources specific to machine learning, but the above, from personal experience, actually was fun in the proper sense: expanding my mind, knowledge, and providing a well thought-out learning experience