Since R is freely available there is quite a bit of educational material available for free as well. While there are dozens online, not all are worth your time. You'll be safe with any (or all) of these.
R For Beginners by Emmanuel Paradis - Excellent book available through CRAN. It's not very long, yet is a good introduction for R. Paradis walks you through how to enter & work with data, introduces packages, plotting & statistical analysis. It also touches on programming. 72 pages.
R Tips by Paul E. Johnson - Another excellent book introducing the major concepts of working with R. The content is very similar to R for Beginners, but the presentation is a little different. Johnson also gives a nice introduction to regression & model fitting. Strongly recommended. 72 pages.
Introduction to Probability & Statistics Using R by G. Jay Kerns - This is a textbook for an undergraduate course in probability & statistics. The (physical) book is available for purchase, or you can download a copy of it for free. The great thing about this one is that all of Kerns' code is downloadable (from GitHub), which makse it very easy to follow along with his examples.
Statistics With R by Vincent Zoonekynd - This is a complete introduction, yet goes quite a bit further into the capabilities of R. Zoonekynd includes clustering, principal component analysis, ANOVA, graphics & plotting, probability distributions, regression & more. The book is available online via HTML, or downloadable as a PDF.
Programming R - This one isn't a downloadable PDF, its a collection of wiki pages focused on R. The book assumes some knowledge of statistics and is focused more on programming so you'll need to have an understanding of the underlying principles. The book is well written, the sample code is clearly explained, and the material is generally easy to follow.
R Inferno by Patrick Burns - This is a great introduction to several aspects of R programming. Loading data into R, iterating over data with loops & logic statements, he even touches on creating packages. This is a complete and thorough book on R. A hard copy is available for purchase through the site, or the entire book is available for download as a PDF. 126 pages.