Our World Religion class is now studying Islam, and, of course, a central topic in Islam is the prophet Muhammad. I found two books that are great for this age range in helping to understand this important man.
The first is the picture book Muhammad by Demi. Demi, a children's author and illustrator whose work mostly centers on ethnic folk tales and/or religious stories or people, has another winner with this book. As with all of Demi's work, the pictures are gorgeous and reflect the style of the culture of the person/story (in this case, Arabic). For example, in deference to the Muslim prohibition against having pictures of Muhammad (it constitutes a "false idol" in their religion), Demi puts him in the picture only as an outline, usually filled in with gold leaf.
But while it is a picture book, it also has plenty of substance for the middle school student. The book provides a good overview of Muhammad's life and role in Islam and Arabic culture. In our case, we read the book aloud, which kept the students absorbed in the story and left time in a single class for some good discussion as well (in our case, we also did a Venn diagram to discuss the similarities and difference between Jesus--the last prophet we studied--and Muhammad).
For more depth, however, I recommend the book Muhammad of Mecca: Prophet of Islam by Elsa Marston. This book goes into more depth about the events in Muhammad's life and tries to keep an even-handed approach about what is known and what is more conjecture. It does not deal with spiritual issues at all, but merely reports the best facts as we know them. However, I thought it was very helpful in explaining the culture of the times, which I think helps explain some of the practices in Islam that can be most confusing or difficult for students to understand (or even Christians/Americans in general). And it is geared toward the middle school level of reading/understanding.
We are using this two books as major building blocks as we try to create our foundation for our study of Islam.