Mastering Shiny, by Hadley Wickham
Happy Git and GitHub for the useR, by Jenny Bryan
R for Data Science, by Hadley Wickham
Hitchhiker’s Guide to Python, by Kenneth Reitz & Tanya Schlusser
Data wrangling essentials: comparisons in JavaScript, Python, SQL, R, and Excel, by Allison Horst & Paul Buffa
W3Schools, particularly for their HTML & CSS tutorials