Advanced R by Hadley Wickham

Advanced R is a free online book that teaches intermediate and advanced programming techniques in R. Written by Hadley Wickham, a prominent statistician and R developer, it covers functional programming, object systems, metaprogramming, and performance optimisation. The book is designed for people who already know R basics and want to understand how the language works under the surface. Rather than offering graphical interfaces or automated analysis, it provides detailed explanations and code examples you can work through yourself. It's particularly valuable for data scientists, statisticians, and software engineers who need to write more efficient R code or understand R's design principles.