Update & Rstats book added (#3906)

free-programming-books.md

@@ -2373,6 +2373,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [An Introduction to Statistical Learning with Applications in R](http://www-bcf.usc.edu/~gareth/ISL/) - Gareth James, Daniela Witten, Trevor Hastie and Robert Tibshirani (PDF)
 * [Cookbook for R](http://www.cookbook-r.com) - Winston Chang
 * [Data Analysis for the Life Sciences](https://leanpub.com/dataanalysisforthelifesciences) - Rafael A Irizarry, Michael I Love *(Leanpub account or valid email requested)*
+* [Efficient R programming](https://csgillespie.github.io/efficientR/) - Colin Gillespie, Robin Lovelace
 * [Exploratory Data Analysis with R](https://leanpub.com/exdata) - Roger D. Peng *(Leanpub account or valid email requested)*
 * [Introduction to Probability and Statistics Using R](https://github.com/gjkerns/IPSUR) - G. Jay Kerns (PDF)
 * [Learning Statistics with R](https://learningstatisticswithr.com/book/) - Danielle Navarro