Book Creator was recently updated for Chrome OS and the Web. You can now insert images from Google Images, merge ebooks, and save epubs offline.
The ability to insert images from Google is a big plus. The Google Images are safe search enabled so you full permissions to use them in your books. Since you currently can't increase font sizes for titles, searching for individual letters is a great way to make a flashy title.
The ability to merge books is also a nice feature. You can have students work on their own page or section of a book and then later merge them altogether in a class book.
The ability to save an epub offline is also a great addition.