Photo by Sylvia Sammon-Burns - Taken at The Common Man Inn
Here are the classic Machine Learning book references.
As a sign of the Academic nature of ML, all are essentially PDF with the intent that the reader would print them out and read them offline. I did that once, and then I lost patience and wrote some code.
As a practitioner, classification is the most useful aspect of Machine Learning in the problems I encounter. Often I want to know group rows in the database according to the data I have with similar important properties - like potential customer value.