Get both unless you're only getting one, in which case, get Shai Shalev-Schwartz's.
[0] http://www.cs.huji.ac.il/~shais/UnderstandingMachineLearning...
[1] https://mitpress.mit.edu/books/foundations-machine-learning
I simply clarified that the question was about computational learning theory, a subfield largely started by Leslie Valiant in the form of PAC (Probably Approximately Correct) learning. The difference in emphasis between the machine learning conferences I mentioned helps point out how practical machine learning (like ICML, matching PRML/ML/ESL) and feature extraction/representation learning (like ICLR, perhaps matching portions of both ICML and ICLR), while important, are not what the previous poster was asking about.