Advanced R Progamming by Hadley Wickham

This book-in-progress has the best explanations of the syntax of R that I have seen. For example:

- R has no scalar, or 0-dimensional, types. All scalars (single numbers or strings) are length-one vectors (http://adv-r.had.co.nz/Data-structures.html).

- For example, library() and require() allow you to call them either with or without quotes, because internally they use deparse(substitute(x)) plus a couple of tricks (http://adv-r.had.co.nz/Computing-on-the-language.html).

- The various R OO Systems are described, compared and contrasted (http://adv-r.had.co.nz/OO-essentials.html).

Excellent book.