I haven't read them, but: Ohlig has taken quite a fringe position along with Muhammad Sven Kalisch and some other scholars, mostly in Germany, that Muhammad didn't exist and/or that the Qur'an either is or began as a Christian text of some kind with a significant influence from Syriac. Christoph Luxenberg (a pseudonym) is perhaps the most noted proponent of the latter idea. Ohlig argues for a "late" Qur'an and Islam, in the 8th century, as opposed to the 7th.

His "Early Islam" for instance, is an essay collection of writings from himself, Luxenberg, Volker Popp (who also holds that Muhammad didn't exist) and Ignaz Goldziher, who is a seminal figure in Islamic Studies but who died 100 years ago.

If you're looking for introductory texts on early Islam or want to go into the historiography of why the vast majority of the field holds these views to be incorrect I would strongly recommend you look elsewhere.

The essay collection "The Qur'an In Its Historical Context" edited by Gabriel Said Edwards, would be a good start on some of the latest historiography of these questions (i.e. from the past decade.) On the historiography of Muhammad himself, I think the best introduction is Robert Hoyland's essay Writing the Biography of the Prophet Muhammad: Problems and Solutions.

As for early Islam, I would look to either Hoyland's In God's Path and Fred Donner's Muhammad and the Believers. The thesis of the latter is controversial, and Donner's own review of Hoyland's book was quite scathing (if, I think, rather petty), but I think you'd be better served than by someone arguing for a fringe position like Ohlig.