One book that I would suggest to anyone is Introduction to Automata Theory, Languages, and Computation - HMU. It is very approachable and presents some very interesting topics (so you won't write a regex for matching HTML and will learn what P vs NP means). On a more practical side, I think that a must read for machine learning is Tom Mitchell - Machine Learning . Another book that from what I've heard is easier to digest is Data Mining: Practical Machine Learning Tools and Techniques.
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