Unusual books for kids

My kids (6 year old boy, 10 year old girl) have recently been much taken by me reading aloud old Epics to them (The Epic of Gilgamesh, Sir Gawain and the Green Knight and The Nibelungenlied). Has anyone else found other good books that you wouldn't think of as kids books that their kids have taken to?

When my kids were still in the picture-book stage, they used to really enjoy looking at art books with me, especially the impressionists ... on just a slight tangent, I also recommend Julie Merberg's books, like A picnic with Monet and A magical day with Matisse, although these are actual board books for kids, illustrated with the artists' works.

In December my 10 year old read and really enjoyed A Christmas Carol. I thought that was a little unusual.
My five year old is currently fixated with a book about Alexis Romanov, son of Tsar Nicholas II.

You know, the other thing I now think of that my 6 year old really enjoys is when I read to him from various manuals for assembling or building things. But, that falls outside the realm of literature! The art books are great, I should pull out some of those.