>Are there any resources explaining the inner workings of the language that I could be referred to? :)
A lot of people will suggest Hadley Wickham's books--and they're generally good! But it also sounds like you would be interested in Programming with R by Chambers. If that works for you, then other works on S/R by Chambers, Becker, or Wilks might be of interest. Statistical Models in S by Chambers & Hastie still has value too, especially given what you're trying to learn.
>Any pro tips for how to progress in my learning?
Work through the manuals, especially An Introduction to R then you'll learn a lot. You can also try R for Beginners . If you feel like you have a grasp on most of that, R Infero is worth a read or at least a skim. If, after all that, you want more then consider Chambers' Programming in R