I just learned that Fieldstone Alliance is having a winter book sale. 30% off all titles, and since all of those titles are focused on nonprofits, it's something you should take a look at.
Go here to learn the details.
Truth in advertising...I have two Fieldstone titles: Nonprofit Stewardship: A Better Way to Lead Your Mission-Based Organization, and Generations: The Challenge of A Lifetime for Your Nonprofit.
That said, there are dozens of great books for sale. Check it out.