What is the ruling on marrying a man from among the People of the Book? And is it obligatory to wait until the first husband expresses his desire to return, or is it permissible to marry another man after his conversion to Islam?

If a disbeliever embraces Islam, he has the rights of Muslims and is bound by their obligations. It is permissible for a Muslim woman to marry him. However, the Muslim woman should not marry the man merely upon his utterance of the Shahada (declaration of faith). Rather, she should wait until she is certain that he has truly embraced Islam and that his Islam is sound. If he embraces Islam and she has to choose between him and her first husband, she should choose the one who is stronger in faith and better in character. If they are equal, then her return to her first husband is preferred for the sake of the child.