When researching the law of nonprofit organizations, keep in mind that you may be researching state and/or federal law depending on the issue(s) you're searching.
Organizations obtain nonprofit status from states, and state law governs the formation and governance of nonprofits. (The corporate form may also be called "benefit" or "nonstock" corporations in some states.) In contrast, the federal Internal Revenue Service (IRS) grants tax-exemption, although organizations may also apply for exemption from state and local taxes as well.
Also note that while most people think of charities when they think of nonprofits, charitable organizations are only one of the many kinds of nonprofit organizations that can request tax exempt status under section 501(c) of the Internal Revenue Code.
The below sites give information such as links to 990 forms, and financial information about nonprofits. Additional information is often available through state agencies. For example, articles of incorporation are generally available through the Secretary of State in California, or with whichever agency a charity has to register with to incorporate in other states.
Many states also require charitable nonprofits to submit documents such as bylaws. While these can be requested from agencies (or the charities themselves), Massachusetts and New York's filing searches include images of these documents: