For people who want to practice machine learning, An Introduction to Statistical Learning with Applications in R by Gareth James, Daniela Witten, Trevor Hastie and Robert Tibshirani is a popular textbook choice. It certainly helps that a PDF version of the book is available for free on the authors’ website, but the book itself is also an excellent introduction to techniques for statistical learning. When refreshing my statistical learning skills earlier this year and learning Python for data analysis, the ISLR book was the main reading in the online course that I took.