These were the books I originally recommended as resources in my own book, The Principles of Project Management:

• Gary Heerken’s Project Management: 24 Steps to Help You Manage Any Project (good refresher)
• Ken Schwaber’s Agile Project Management with SCRUM (for PM’s managing software projects)
• Scott Berkun’s The Art of Project Management (for learning the finesse which is the hallmark of truly great project managers)
• Project Management Institute’s Guide to the PMBOK (for hard core theory – not for the faint-hearted!)
• David Allen’s Getting Things Done: The Art of Stress-Free Productivity (step change your prioritization & personal productivity)
• Tom DeMarco & Timothy Lister’s Peopleware: Productive Projects & Teams (understanding the people side of projects & software)
• Michael Lopp’s Managing Humans (trials, tribulations & pitfalls of people management — worth reading especially because it is absolutely hilarious)