A good book on Monarch butterflies is An Extraordinary Life: The Story of a Monarch Butterfly by Laurence Pringle. The text is engaging and the illustrations are wonderful. It tells the story of the life cycle of the Monarch, including the migration to Mexico. There is a section at the end of the book about saving the butterflies habitat in the tropics, but that can be easily skipped if yoy prefer. The book is out of print, though, so you will have to see if you can get a copy from the library. There are several people on Amazon selling used copies for about $20, also.