Free R Books
Recommended on Twitter
http://r4ds.had.co.nz/ - R for Data Science
http://adv-r.had.co.nz/ - Advanced R
http://r-pkgs.had.co.nz/
http://moderndive.com/ - recommended by Jesse
http://happygitwithr.com/
https://www.tidytextmining.com/
last updated september 2019