The following books emphasize the language features of R (opposed to applications such as statistics, machine learning or visualization).
|Norman Matloff, The Art of R Programming: A Tour of Statistical Software Design, No Starch Press; 1 edition (October 15, 2011)|
|Hadley Wickham, Advanced R, Chapman and Hall/CRC; 1 edition (September 25, 2014)|
|John M. Chambers, Extending R, Chapman and Hall/CRC (May 24, 2016)|
|Hadley Wickham, R Packages: Organize, Test, Document, and Share Your Code, O'Reilly Media; 1 edition (April 13, 2015)|
Dirk Eddelbuettel, Seamless R and C++ Integration with Rcpp, Springer; 2013 edition (June 4, 2013)