I just came across a wonderful book in Andrew's Bookshop, in my local shopping street. It's called The Mutt Book. It has extensive photos of all kinds of mutts with clear instructions for assessing features that might point to the origins of your dog. For instance, it has pages with photos of different body shapes, ears, muzzles, coats and tails, and pointers about how to examine your own dog to check out her background.
I looked through it for ages, trying to decide whether to buy it; Andrew's is the kind of shop where you feel you can browse at your leisure. I put it back on the shelf because I think I'll ask my local library to buy a copy. It's the sort of book that would be a great library acquistion because it is a reference for all of us who wonder about the genetic heritage of our dogs.
At Amazon there is a good example of the book, with plenty of pages to look at. The index doesn't refer to maltese, poodle or shih tzu, which I think are Penny's three ancestries, so that was why I didn't buy it.
On the other hand, I might just wander up to the shop tomorrow and think about it again...