No question, I would censor DS6's book selection at this point. We don't go to the town library often, and it's never come up there-- but if he were to happen upon, say, Samuel R. Delany's "Hogg", it would not be coming home with us.

(Dear reader, resist your impulse to search if you have a weak stomach.)
We have a fair number of books in our home written for adults (many still in boxes after the last move which was not very recent

), but so far the ones he's picked out haven't been an issue. I also buy books specifically for him, which can include some challenging material at times, and he's free of course to choose among those. I would guess that I'm a fair bit looser than most parents would be in terms of restricting his access to content, but I still draw the line at certain topics, for example suicide (though self-sacrifice for others is fine) and sex (especially any mention of graphic or kinky sex, though characters kissing and winding up in bed with each other could be all right depending on the context).