Need Software to write a book

My 7th grader is writing a book for a class assignment.  He is a Harry
Potter fan, so he is planning a very long, chapter book, illustrated
with his drawings.

I am very happy about his industrious plan, and I told him I would see
if there was any software out there for him to use to write his book. 
Does anyone have any suggestions?

I understand that he should just write it out, and then worry about
the page by page format, but he is interested in seeing it lay out on
a book-like format as he goes. And since he seems to be so interested
in doing a project that is way above and beyond the assignment, I am
interested in supporting him so he can do it in the most fun way
possible, if that makes any sense.