A friend who enjoys perverse Americana sent me to a website with "lesbian" paperback artwork from 1950s and '60s pulp fiction. Lust on the Run ("He forgot his tramp wife in a depraved love-nest"), by Josh Tithing, sold for 95 cents -- but only to adults. The frontal nudity on the cover is obscured by "Adults Only" and "Sale to Minors Forbidden" warnings.
While movies, music and magazines frequently bear similar cautions, with legislatures enforcing them by statute, I've never seen a book in a bookstore that had even a "parental advisory" sticker. Romance novel fansites will rate books for the level of sexual content, but this information must be sought out; it does not appear on the cover of explicit works.
Texas, like most states, prohibits the sale, distribution or display of harmful material to a minor, with the standard definitions of those term (although I didn't know that prurient interest in excretion was so widespread as to have been noticed by the Legislature). "Harmful material" is distinguished from banned obscenity by the specification that it be "patently offensive to prevailing standards in the adult community as a whole with respect to what is suitable for minors; and utterly without redeeming social value for minors." These definitions imply that materials can be legal for adults but illegal for children.
In order to be prosecuted, the seller must know that the material is harmful, which poses something of a problem regarding the vast majority of books. Movies are rated, magazines shrink-wrapped, CDs labelled, but I don't know of any voluntary or statutorily-required system that warns librarians or booksellers away from pornographic books.
The warnings on the cover of those lesbian paperbacks likely served the same dual function that "R" ratings now do: yes, this material is inappropriate for children, but that just proves how enticing it will be for adults (and minors who get past the restriction).