Some links from the IRS
Some additional links
The following is older material but may still be useful
An article on this blog titled The Elusive 990.
Some other web resources.
How to Read the IRS Form 990
& Find Out What it Means
How to Read IRS Form 990 from Fidelity
A variety of nonprofit financial links via SNPO
Assessing Nonprofit Financial Measures from Nonprofit Research Fund