If she's not too squicked out by biology, there's a book showing a dissection of a giant squid called Outside and Inside GIANT SQUID by Sandra Markle. If octopus books are of interest, too, there's Gentle Giant Octopus by Karen Wallace and Mike Bostock.
That's about all I can think of that would be accessible to a 6-year-old, although there are a lot of other books with lots of pictures where you could summarize the text that she wouldn't understand, Mark Norman's Cephalopods: a World Guide comes to mind, as does The Deep by Claire Nouvian. Nancy & Colin's book has a lot of good pictures, too, of course, even though it's primarily aimed at ceph care.
I assume you don't have an actual tank of live squid in your living room, so I'm curious what you mean, too....