R. Becker 
New S Language [PDF ebook] 

This book provides documentation for a new version of the S system released in 1988. The new S enhances the features that have made S popular: interactive computing, flexible graphics, data management and a large collection of functions. The new S features make possible new applications and higher-level programming, including a single unified language, user defined functions as first-class objects, symbolic computations, more accurate numerical calculations and a new approach to graphics. S now provides direct interfaces to the poowerful tool of the UNIX operating system and to algorithms implemented in Fortran and C.