History of the Qur'anic Text from Revelation to Compilation: A Comparative Study with Old and New Testaments by Muhammad Mustafa al-Azami
A detailed history of the Qur'anic preservation, step by step and explaining all the points of interest. Makes a comparison of the Qur'anic preservation with that of the Bible. Also answers and busts the myths created by Orientalists about the textual stability of the Qur'an. If there has to be just one book on this subject, here it is.