"Peter Duck" by Arthur Ransome

This book has a very unusual premise:  it is a fictional story written all together by the (also fictional) characters of Swallows and Amazons and Swallowdale .  It's a story that they made up together that stars themselves, but also a bunch of people they made up, like Peter Duck himself.  Got that straight, now?  If not, don't fret -- it took us a little while to get our minds wrapped around it too.  But we did, eventually!  
I read this aloud to my kids over the summer, though we got sidetracke...