Don't copy, imitate. The book was very easy to write. Just lay out 52 exercises, 26 for super basic stuff, 26 for progressively more advanced stuff. Take about an hour or two every night and write a couple. Pretty simple really. You may not have my style in your writing, but you'll definitely get a book that is more Ruby than what I've written.

One warning though, if you think it'll be better to have the book crowdsourced, think again. Programmers seem to be pretty bad at teaching programming, so it's best to just write your book and have people give you feedback as you go.