Inspire Infekt
Believe it or not, most muslims will tell you they believe in jesus and that we worship the same God as Christians. And as long as the book is factual and not misleading, I don't mind that the author is Muslim.
The article has been updated by the author's side of the story - interesting.
If he's a repented Christian and/or follows that actual facts about the life of Jesus, then no - it doesn't matter if he's Muslim or was a Muslim.
If he writes a book that is written in order to make others fall by believing false facts, that's when it becomes an issue. (which was the original concern).