So let's just say this right out front -- I'm not in favor of book banning. But, truth is, I don't think most people are. I believe that there are a number of other approaches to books that require analysis but aren't as absolute as forbidding a book's being published, or collecting books that exist and eliminating their presence from the greater universe.
I think that if you were to get together a large group of people who are "liberal" minded, whatever that means, and ask them if it is appropriate to hand an 8 year old "Tropic of Cancer" most would agree that the book is not intended for, nor healthy for a child of that age. We can agree that some things are appropriate for people of a certain age. For the most part, people won't object to the "voluntary" rating of movies. (I say "voluntary" because IIRC an unrated movie will have a harder time with its mass-market distribution so most movies made for the public-at-large really HAVE to submit to the MPAA to be commercially viable).
So if we can agree in principle that some things are ok, or are not ok for people of a certain age, then the rest is just a matter of working out details. Trust me -- this sounds horrible and, as a younger man-child I was appalled at the efforts of the PMRC to "rate" music. But as a parent, it has become clear that I would rather my child not hear certain words and concepts masquerading as music so some sort of advisory on the outside, letting me know what the child is getting in to is a good idea. Additionally, content warnings on TV shows (and the ability, theoretically to adjust a V chip or equivalent so that youngsters can't watch stuff), and even age confirmations on web sites, or filters all are good things.
Now, maybe I'm not understanding the attempts made by certain forces so I'm using language differently, but I think that an equivalent rating system on books is desirable. We have quantifiable standards that allow experts to decide what music, tv shows and movies are appropriate for what age. Why not have the same standards established (violence, adult themes, language etc) for print texts? And, yes, then I think that a school library might choose either not to hold a book which would not be something for children to read, or to have the books that are for certain ages be kept in different locations in the library apart from those for more tender eyes.
But shouldn't the parents have the right to decide what their children read? Sure, just as an adult can walk an 8 year old in to an R rated movie, an adult can buy a book, or take it out of the local library. But that doesn't mean that an R rated movie has to be shown in the school. And then the question becomes "why do my taxes support a public library that stocks books X, Y and Z?" and to that, the answer is that we don't have line-itemization in taxes and my money goes to all sorts of local functions and programs that I might not otherwise support. Our libraries don't stock every book -- some things are, indeed, too (insert controversial content here) and librarians and their communities have been making judgments about what to have and not have for a while now.
So, again, I'm against anything akin to a complete ban, but I do think that all stakeholders -- parents, teachers and even politicos can help determine what is properly accessible to whom and what isn't so that in certain contexts. Will we all agree in each case? Can we set certain universals (ideological issues aren't subject to ratings -- we limit to descriptions of violence, sexuality and scatalogical imagery maybe?) as a working-starting point? Will it be easy? Will it be quick? Will it be free?
But does a challenge mean we don't try if there is an underlying point of agreement?