If you want to bring ??? ????? into this, I might sooner say that appropriate books are assur. The Sefer Hachinuch (387) writes:
??? ?? ???? ?? ??? ????? ????? ???? ???? ??? ????? ????? ???? ???? ???? ??? ???? ?????? ??????? ?????? ????? ???? ?????? ??? ??? ?????? ????, ????? ??? ???? ??? ?????? ???? ????? ?????? ?????? ????? ?? ?????? ???? ???????? ?? ?? ???? ???? ???? ?? ???? ?? ??? ?? ???? ??? ?? ???? ??? ??????
In other words (the derasha of ???? ????? ?? ????? ????? ?????? ?? ???? notwithstanding), ??? ????? is not specifically referring to “inappropriate” things; it is referring to any (excessive) pursuit of physical pleasures. The Rambam in Sefer Hamitzvos (47) similarly writes:
????? ???? ???? ????? ?? ????? ????? ????? ????? ??? ?? ???? ?? ???? ???’ ????? ?????? ?? ???? ????? ????? ????? ????? ????? ???? ?? ?? ?? ??? ???? ????? ???? ???? ???? ??? ?????? ??????? ?????? ?????? ???
Also, according to R’ Moshe the issur is specifically ??? ????? ????? ???? ????.
It is perhaps notable that the Pele Yoetz, in the section on nivul peh, speaks severely about speaking and listening to nivul peh but makes no mention of reading it.