Tally's growth in Scott Westerfield's "Uglies" series is also a thing to behold. He can turn her from good guy to bad guy to good and make us buy it totally. The man is phenomenal.
I do crazy things, like reading sequels and trilogies and actually mapping character arcs. I do it for individual books too. I learn a lot that way, probably more than the author intended! I find many series are "flat" in growth, but that seems par for them, they are intended to be serial, not arcing, especially in MG where they are adventure after adventure or comic situation after comic situation.
I plan way way ahead, taping pieces of paper together on the floor and mapping out all the arcs of all the characters over one book and over multiple. The series I've been working on has four critical characters and I've got them overlapping all over the place, which will hopefully help me align the overarching plot with their growth and keep things well paced. I've planned for three MGs and a YA finale as the kids will age from ten to thirteen over the course of the books, and all begin to lose their magic as they go from tween to teen, sparking the final showdown in whether or not we can truly retain our belief in the extraordinary as we grow. To make them shift from not believing, to believing, to not believing, and going separate directions is something to orchestrate, and I fear making mistakes in early books I can't "undo" if I don't plan carefully.
Westerfield's books have been very helpful in this way. I didn't find Harry Potter as useful in that regard, as the characterization feels muddy to me in the middle books. Another awesome pair of books on pure character development has been Sonya Sones (What My Mother Doesn't Know and What My Girlfriend Doesn't Know). She did what I am doing, which is to switch primary characters, letting the previous MC become secondary in the subsequent book, so the new arc can shine even as the old favorites still grow and change in the background.
Character growth is one of my absolute favorite things about writing in the MG and YA arena. Their maturation happens organically, always fresh and exciting as they become the adults they are meant to be, slackers or heroes, sirens or wallflowers, happy or discontent.