Izzy is a great friend who stands by Brooke through all of her sometimes silly adventures, even though her feelings are hurt when Brooke abandons her to hang out with the "popular" girls. They do come to an understanding, and it's good to see that they both understand the other girl even when they don't necessarily get along.
My own daughter, who was a reluctant reader, was completely enamored of Malcolm's The Ruby Princess Runs Away series when she was in about second grade, and we had to hunt down all 12 books PLUS the super special. There is something tremendously comforting and addictive about series books for young readers. If it keeps them reading, I'm all for it. The Wish Fairy series joins the ranks of magical series like Mlynowski's Whatever After, and a huge range of series involving exclusively fairies: Rodda's Fairy Realm, Meadow's Rainbow Magic, Perelman's Candy Fairies, Small's Fairy Animals, and probably many more I haven't read!