Articles
If you only read one article on Engage, make it The Necessary Art of Persuasion by Jay Conger, Harvard Business Review, May/June 1998. You’ll see a good overview of Emotional Intelligence in What Makes a Leader by Daniel Goleman, Harvard Business Review Nov/Dec 1988. And emphasising the importance of relationships is What Effective General Managers Really Do by John Kotter, Harvard Business Review Mar/Apr 1999. All three articles are available at www.hbr.org
Other articles are A leadership style that switches on engagement can make staff radiate productivity by Beverly Alimo-Metcalfe and Margaret Bradley, People Management Jan 2008 and Get 'engaged' by Emma De Vita in Management Today April 2007.
Books
My recommendations have to start with a recent book from my friend and colleague, David Macleod. With Chris Brady, he's written The Extra Mile, How to Engage Your People to Win.
Also consider
On Form by Jim Loehr and Tony Schwartz
Executive EQ by Robert Cooper and Ayman Sawaf, and
Leading Out Loud by Terry Pearce.