I have not read Siljander's book, but I have read material by Nikides in the past and find him to be a competent and fair scholar. One quote from his article that I really liked follows:
The Qur’an is often unfathomable to Christian readers, just as the opposite is true, because it has no narrative, redemptive story organizing the text. With the exception of its first sura, it is simply organized from longest to shortest suras. It lacks the kind of logical flow we find in the Bible. This makes it difficult for Christian readers to understand the Qur’an, but it also makes it difficult for Muslim readers to adjust to the Bible’s underlying structure and logic.
Nikides' review of Siljander's book can be found HERE, in the April issue fo St Francis Magazine.