What Reference Books would you recommend?

Hi All

I was wondering if anyone had some really great plant identification books that they would recommend. While the internet is great, I like to have something that I can just throw in a pack when I'm out and about.

For medicinal herbs etc I've been looking at The Earthwise Herbal from Matthew Wood, both old and new world volumes. I've also heard about Botany in a Day by Thomas J. Elpel

Any others?