Are you talking about primary sources? I quoted pretty much all the early mentions of Muhammad in contemporary Christian sources here and here. You won't be able to find these online, apart from an older translation of Pseudo-Sebeos available here. Most of these sources and some analysis of them however can be found in Robert Hoyland's Seeing Islam as Others Saw It, which is a very useful and insightful book covering this exact issue.

As for secondary sources, Michael Cook's Muhammad would be a nice introduction to early Islam, followed perhaps by Fred Donner's Muhammad and the Believers, a more recent book that proposed a very interesting thesis, it is very readable but don't treat it as the historical 'consensus' on early Islam. For the expansion of Islam, you can't go wrong with reading any books by Walter Kaegi, Fred Donner and Hugh Kennedy, though Kennedy's The Great Arab Conquests is the newest and the most readable (as it was written for the general public rather than academics).