Jennifer Homer, VP, Communications & Career Development, recommends: Multipliers: How the Best Leaders Make Everyone Smarter, by Liz Wiseman.

Nancy Olson, Community of Practice Manager, recommends: The ASTD Leadership Handbook, and Infoline: Critical Competencies for the 21st Century Leaders by Lou Russell

Pat Galagan, Community of Practice Manager, recommends:
Judgment: How Winning Leaders Make Great Calls, by Noel Tichy and Warren Bennis and True North: Discover Your Authentic Leadership, by Bill George. For the Federal leader, The People Factor: Strengthening America By Investing in Public Service, by Linda J. Bilmes and W. Scott Gould.  

Ann Pace recommends: Gary Hamel’s What Matters Now. It’s especially focused on leaders in the 21st century and “management 2.0.”

Ryann Ellis, Learning Circuits editor, recommends:
Peter Drucker. He's still the best. Check out the Peter Drucker Institute Blog if you don’t have time to read all of Drucker’s 30+ books.

Marcus Buckingham’s books on leading by focusing on people’s strengths. E.g. First Break All the Rules: What World’s Greatest Managers Do Differently

HBR Blog Network. As a guest, you get limited access; with a subscription, you can see a great selection of what other people are thinking, reading, following. I also like HBR's 10 Must Reads series that are compilations of favorite articles from people like Kaplan, Kotter, and others.  They have 10 Must Reads for Strategy, 10 Must Reads for Managing People, and so on.

Lisa Bodell’s book, Kill the Company. She argues that complacency can be as bad for organizations as taking too many risks. Her book has lots of small, practical things leaders can do to make big changes. She calls these "Little Bigs." Read my book brief on the ASTD Human Capital blog.