If a teacher, parent, students, or support staff member has a question on library resources, it should always be directed to the divisional librarian (if your school has more than one librarian).
Advocate for your own knowledge regarding the right of all students to access information and promote that part of your job to others in the school.
When a librarian receives a complaint, the first step is a conversation; NOT an email. Make time to call or set up a face to face.
As hard as it can be at times, be patient and listen without judging. Listen and do not agree or disagree. A parent is rarely questioning a book but rather how to deal with hard or unexpected conversations with his/her child.
Try to identify what they are concerned about and offer alternative solutions or points such as:

If the phone call or conversation does not resolve the concern, the next step is to send the parent(s) the 'Reconsideration of Library Materials' form.
In other words, the process becomes more formal. See example below from the ALA (as well as the linked PDF).