I don't know why these books have been challenged or by whom. I do know though that frequently and historically certain types or sects of Christians are in the vanguard of the book banning stakes. Frequently such people are in the vanguard of those who would impede intellectual liberty - particularly in relation to literature and a view of creation based on a narrow interpretation of the Book of Genesis. History is seldom, if ever, on their side and makes a mockery of their attempts to turn back the intellectual tide. Their religious history once demanded a freedom to read and judge a particular book for themselves - but henceforth they are happy to deny such freedom to others.
There is one way to know good literature from bad; good argument from bad - and that is to read widely, explore widely. Then one is able to sift the wheat from the chaff in exactly the same way that those who call themselves Christians will one day be sifted: nutritious grain from sterile chaff.