Hadley Wickham's original tidy data paper: Wickham, Hadley. 2014. “Tidy Data.” Journal of Statistical Software Volume 59 (Issue 10).

Pontifications

NB: It is important to point out that the reshape2 package which includes melt and cast has been superseded by the tidyr package which includes gather and spread!
But the original paper is still worth reading: Wickham, Hadley. 2014. “Tidy Data.” Journal of Statistical Software Volume 59 (Issue 10) at some point (I bet it is summarized/explained even better in one of the books that Wickham wrote that I already bought like the R for Data Science book:-) !)