Update:

14-02-2021, the book's webpage http://www.network-theory.co.uk/R/base/ is no longer online. We switched the download link to a documentation of the R Base Package.

Book excerpts:

This manual is the first volume of the complete reference manual for the base package of GNU R, a free software environment for statistical computing and graphics. This volume documents the core commands in the base package of R. These commands include programming constructs, basic numerical functions and system calls. The base package commands are automatically available when the R environment is started.

The second companion volume 'The R Reference Manual - Base Package - Volume 2' (ISBN 0-9546120-1-9) contains documentation for commands for graphics, mathematics, distributions and random numbers, models, time-series and datasets. Together the first and second volumes document the complete base package of R (version 1.8.1).

All the reference manuals are written by the developers of R, and represent the definitive guide to the software. The manuals explain the use of each command in detail, with references, implementation notes and examples. Every volume includes a comprehensive index of entries listed by keyword and topic.

NOTE: You need both volumes for the complete documentation. The individual volumes are not intended to be used on their own.