I'm not sure it's really an issue of how easy it is or is not; as Linda says, it's probably more what people are asking for.
In Manage Content, there is a 'read' tag, now on anything you've read; I'm not sure it would help me much to be able to sort to include or exclude those books. AND ... I am pretty sure there are some omissions. I'm feel like there are some I have read -- before they introduced that feature -- that are NOT marked as read. ON my kindle, I tend to remove books once I have read them.
I don't see any value for me in sorting by how much of a thing I've read. Sorting by length of the book could be useful when I'm looking for something shorter. But if I switch to list view I can see that information in a relative way, which is good enough for me.
But, yeah .... if it's something YOU would be interested in, you should let them know.
