What do R-rated and X-rated mean here?

Solution 1:

To interpret what is meant here, I think you have to consider a synonym of X-rated: hard core.

I would think that an “X-rated” book on stochastic modeling, then, would be “hard core.” In other words: chock full of intense math, and not very friendly toward the casual hobbyist. Another way to say this might be, “not for the faint of heart.” I can imagine someone issuing a warning: “Don't buy this book if you skipped the prerequisite course.” I'd expect such a text to be on the opposite side of the spectrum of where I'd find Math for Dummies, which, to extend the metaphor, would be a G-rated book.

Solution 2:

Yes it's a joke on the R (restricted) and X (adult) cinema classifications.

It means the first two books are moderately complicated and the last two are much more intimidating. The term hard-core, also from the adult entertainment industry, is also used.