Bit late to the party, but here might be another reason for this error (and solution):
If you are using cmake, make sure to add the include file with the QT object ( AddressBook.h in the question) to the target-sources: eg at add_executable.
Else the MOC will not process that include file. (if it is header-only).
(Note: normally for include files it is not needed to add them to the cmake target, so this mistake is easy to make)