If you're looking for the tax requirements for your nonprofit, this page on the IRS website is a good place to start. Exemption, unrelated business income, employment taxes, filing requirements, etc.
Nolo.com has provided some free legal and management articles for nonprofits. They are intuitively displayed and clearly written. There are approximately 20 articles.
Non-profit or For-Profit? Basic Considerations [USA and Canada]
How to Form a 501(c)(3) Nonprofit Corporation
Hurwit & Associates State-by-State Filing Requirements Tool
CMC Toolkit