Any suggestions for good references on the study of natural shapes ?
I have a great book called "Patterns in Nature" by Peter S. Stevens, but I'd like to go more in-depth ; specially on the subject of shells and skeletons.
the book you're starting from is really nice, let me suggest you some books that I find fundamental for the topic you're up to:
. On Growth and Form, D'Arcy Wentworth Thompson
. Structure in Nature is a Strategy for Design, Peter Pearce (yes I agree that when he goes on polyhedra derived projects things get outdated but I find the first part really helpful)
. Kunstformen der Natur, Ernst Haeckel
You can also google them, you'll find lots of resources: