This branch of the American Library Association has ready access to reviews and resources to answer questions about raised about a specific title. A request for reconsideration form is also a necessity. When the person making the complaint isn't satisfied with reviews or reassurances then they have the right to make a formal complaint. Allow the person to take the request for reconsideration form and complete it. No action regarding a book should be taken until request for reconsideration is completed and returned.
Books are to be let on the shelf, in circulation.
You should also have in place a formal Collection Development Policy, which addresses controversial materials. The Supreme Court has ruled that no one, librarian, principal, mayor, etc., the right to supercede an approved request for reconsideration policy. A challenge of your materials can be scary, nerve wracking, and infuriating. Remember that parents have the right to direct what their own children read. They do not have the automatic right to determine what your or my children read.