The Preaching of Islam is a book by T.W. Arnold and it is a must read. Here is some background on it.
First Edition : Aligarh, 1896
Second Edition : London, 1913
Here are the Chapter titles :
Muhammed (PBUH) As A Preacher of Islam
Spread of Islam Among Christian Nations of Western Asia
Spread of Islam Among Christian Nations of Africa.
Spread of Islam Among Christian of Spain
Spread of Islam Among Christian Nations of Europe Under The Turks
Spread of Islam in Persia and Central Asia
Spread of Islam Among the Mongols and Taters
Spread of Islam in India
Spread of Islam in China
Spread of Islam in Africa.
Spread of Islam in Malay Archipelago
To write it he benefited from libraries all over the world and particularly the personal libraries of the Nawabs. The references include books in more than a dozen languages including English, French, German, Russian, Greek. Though the author says that this is not a book about the conquests and battles but anyway he ends up thoroughly demolishing the popular belief that Islam was spread with the power of the sword. For every region in which Islam is found the only mode of spread is either preaching by the pious or the character and the way of life of Muslim traders. The Theory of sword is dead, for long. Only if we new it!