XmlSerializer and circular reference

Question

I have class book and in that class I have declared a customer object pointing to null.

When the customer-object isn´t pointing to null I think I get a circular reference error.

I also have another object in which a have a List. If the List is empty it´s no problem but when it is containing objects I  get circular reference.

Not sure which is the problem or both.

Thanks to Vulpes for helping me with serielization.