When I first started using R, my code looked just like MATLAB code. I read an intro to R programming, did some data analysis using R, and I was using the arrow assignment operator! But otherwise, my R code looked very much like MATLAB. Then I read Advanced R by Hadley Wickham. I found out, while I wasn't doing anything wrong, I wasn't getting the most out of R.
Advanced R is very readable if you've used R and maybe some other programming experience. If you're self-taught in R and have no other programming experience, you may find the book a bit tough. The book does a great job describing the important features and how to make the most of them. I particularly liked the chapter on functions and the section on functional programming.
I highly recommend it!
Update: A commenters pointed out there is a new version of the book out. The link is below.