The Right To Read: Censorship in the School Library. This is the text of an old (from 1990) but still timely ERIC Digest.
Almost all librarians agree that censorship is a bad thing. Censorship can occur when library materials are challenged by the community. It can also happen when librarians make acquisition decisions.
It is important to remember that this censorship is not just a right-wing fundamentalist endeavor. Many groups (from all over the political spectrum) attempt censorship. The site noted, "Those who lead library censorship campaigns come from both right- and left-wing organizations. Fundamentalists may dislike the open discussion of such issues as abortion in news magazines, feminists may challenge outdated female stereotypes found in certain books, and African-American groups may object to the portrayal of members of their race in such works as Huckleberry Finn or the Black Sambo stories."
I think we hear more about conservative censorship than liberal censorship due to the composition of the library profession. Most librarians consider themselves to be progressives. When they are challenged by a right-wing group, these librarians immediately publicize the incident. When they are challenged by a left-wing group, they keep it quiet and are more likely to find an accommodation without actually engaging in censorship.
This subtle bias is probably evident in purchasing decisions too. How many children's librarians are actively seeking out children's books from religious publishers that those from the left would find objectionable? How many of these same librarians are buying every children's book that comes out portraying homosexuality and families? Hence, the very nature of what is bought is likely to dictate what books get challenged by which groups.
This would make a fascinating study. I am kicking around dissertation topics right now (the time is growing close!) and censorship by librarians via purchasing decisions might make for a good topic. It would be easy to get into OCLC and actually compare the ownership of select titles and see if there is a visible ideological difference in selection rates. It may give Banned Books Week a new twist.