Philip K. Hitti writes (p.112):
The first record of [Mohammed's] life was undertaken by ibn-Ishaq, who died in Baghdad about A.H. 150 (767), and whose biography of the Prophet has been preserved only in the later recension of ibn-Hisham, who died in Egypt about A.H. 218 (833). Other than Arabic sources for the life of the Prophet and the early period of nascent Islam we have none. The first Byzantine chronicler to record some facts about “the ruler of the Saracens and the pseudo-prophet” was Theophanis  in the early part of the ninth century. The first reference to Muhammed in Syriac occurs in a seventh century work. 
1. Chronographia, ed. Carolus de Boor (Leipzig, 1885), p. 333. [Here]
2. A. Mingana, Sources Syriaques, vol. 1, Bar-Penkaye (Leipzig, 1908), p.146 (text) = p. 175 (tr.)
I’m not sure that Hitti is complete. Jacob of Edessa certainly mentions Mohammed briefly in his chronicle, although the way in which this was published in the CSCO edition would disguise this from most people.
But I was wondering what was online of all this. Naturally I turned to Wikipedia, which had an article on ibn Hisham. This links to an English translation now only available in Archive.org, since the original site has disappeared. It is here. But… what is this, I wondered? Is it out of copyright? When was it done?
There is an introduction by “Michael Edwardes”. A certain amount of hunting around reveals that the book is in fact “The life of Muhammad, Apostle of Allah”, edited by Michael Edwardes, London: Folio Society (1964); Selections from Edward Rehatsek’s translation of Sīrat Rasūl Allāh by Ibn Isḥāq in the recension of Ibn Hishām. Some details of the book is here:
This translation of his biography of Muhammad, published here for the first time, was made by a Hungarian, Edward Rehatsek, who spent most of his life in India. He presented the manuscript of his translation to the Royal Asiatic Society of London, and it is now published courtesy of the Society. …
Ed Jajko comments on my review of Ibn Ishaq, The Life of Muhammad, Apostle of Allah. Edited by Michael Edwardes: “Sirat rasul Allah by Muhammad ibn Ishaq is indeed the earliest extant biography of the prophet Muhammad, and thus an extremely important work. Not to deny anyone the pleasure of enjoying the new Folio Society volume, I would like to point out that this is not a new or a complete edition and that there are other ways of accessing the information. Michael Edwardes first published the Rehatsek translation in 1890 or 1894. Edwardes abridged Rehatsek’s translation for the general reader. …
In the introduction, Edwardes writes:
“The translation which follows is the first known English version of Ibn Ishaq’s biography, and is here published for the first time. The translator, Edward Rehatsek, was born in Hungary in 1819 and died in Bombay in 1891. He arrived in India in 1847 and spent a number of years in research upon oriental subjects. He later became professor of mathematics and Latin at Wilson College, Bombay, from which position he retired in 1871. Rehatsek lived the life of a recluse, working upon his translations from Arabic and many other languages. After his death, his body was burned in the Hindu manner, the first European, it is said, to be cremated in India. The manuscript of the translation was completed just before his death and was presented to the Royal Asiatic Society, London, by F. F. Arbuthnot, the Islamic scholar, in 1898. This edition is published by courtesy of the Society.
The original work is extremely long, over a thousand pages of the translator’s small yet clear handwriting. Rehatsek produced an almost literal translation and it suffers somewhat from scholarly pedanticism. In preparing this edition for publication, I have kept one main aim in view – to present the earliest extant life of Muhammad in a form, and at a length, acceptable to the general reader. To do this it has been necessary to cut the text as well as to make some rearrangement in the interests of orderly chronology. I have inserted linking passages, printed in italic, where the text seems to require it. Generally speaking, those parts which have been excised have been repetitions of events, long lists of names, confusing accounts of minor battles, and a large quantity of verse. Some errors have been corrected and verbal infelicities removed.
Most of us will shudder at the cavalier statements of Edwardes, but doubtless he worked under the orders of a publisher who had somehow to sell the book to the public.
But how accurate are the statements? Rehatsek certainly lived in the 19th century. But Edwardes? He looks like a 60’s figure, from online catalogues. Was Rehatsek ever published before 1964? If his translation exists in manuscript still, perhaps we would all be best served by a transcription, if it is public domain (rather a lot of ‘if’s there!) It is certainly out of copyright in the US, but I find it hard to tell whether an unpublished manuscript would still be in copyright in the UK, although Rehatsek died more than 70 years ago.
Another Wikipedia article tells us that a modern translation of the complete ibn Hisham does exist, by Alfred Guillaume, The Life of Muhammad. A translation of Ishaq’s “Sirat Rasul Allah”, with introduction [xiii-xliii] and notes (Oxford University 1955), xlvii + 815 pages.
So what’s the upshot? Ibn Isham is not online. Excerpts from him are, in a dubiously legal way, dubiously edited. It would seem that we could all do with much better materials online for early Islamic history.