Michael Erard (website) has just recently published a new book which gives considerable attention to um and uh and other hesitation phenomena. The title is Um … Slips, Stumbles, and Verbal Blunders, And What They Mean … Here's a bit from the website:

You make thousands of verbal blunders every day. They mean more than you think.

Um... is the first book to tell you how.

The book is recommended by Geoffrey Nunberg of UC-Berkeley as well as other linguists who post regularly at (*the) Language Log.