I'm guessing eskielover meant sexually explicit (possibly too graphic for a 11-year-old), not literally illicit (not even allowable in the library because clearly obscene by prevailing community standards).

I was wondering if what's considered suitable or unsuitable for minors in that community is left up to the librarians' judgment; if it's between the librarians and the parents; if there are local guidelines already in place that the librarian was either honoring or overstepping; and if state law already dictates what libraries (and booksellers, and schools, and maybe even parents) can and can't let minors see. Not knowing any of this, we may not be in a good position to speculate on what the librarian should have done. For a discussion of some of the legal considerations (freedom of speech/press versus community standards), check Obscenity (Wikipedia).

I happen to think that two important factors in this particular case would've been the "redeeming social importance", if any, of the book in question and how the kid was planning to use it. Ideally, the kid would already have been getting age-appropriate sex education from his/her parents but we all know that doesn't always happen. When it doesn't, it's an open question whether kids are better left ignorant too long or told too much too soon. They're pretty sure to learn stuff from each other anyway, and what they learn by that route may or may not compare well to what they're being protected from learning from library books.

I think the results would depend a lot on what kind of relationship the librarian already had with the kid and on how much time the librarian had to follow up properly. If there were several other people lined up at the desk and probably listening, the kid could conceivably rush out the door in embarrassment and never come back. Even if they found a few minutes for a serious conversation, the kid could offer some answer that the librarian wasn't sure what to make of or how best to follow up.

I'm more than a little leery of saying that "of course" whole categories of materials should be kept from children "for their own good", because in the past I've seen similar arguments used for keeping other, socially important materials from adults as well.

For more discussion see Sex Education (Wikipedia), where I found this picture: