I am recommending this revised and expanded edition of "The Ukulele, A Visual History" by Jim Beloff (Backbeat Books, S.F., CA 2003). It has the history of the uke, the players, the manufacturers and the story. There are tons of photos of cool and crazy-looking ukes to please the collector, plus many vintage graphics in a beautifully layed-out book.
Back Beat books has some interesting books on other musical subjects as well. And here's a great uke site: Ukulelia
The Art of Amy Crehore
The Art of Amy Crehore