The handbook, what has changed since it was printed, and Google's own pages on the program.
Free to read in your account, with printable checklists and templates.
Google changes the Ad Grants rules from time to time. We re-check Google's pages at least every 90 days. When something changes, it's listed here with the date, what changed, the chapter it affects, and a link to Google's page.
The policies themselves, from Google for Nonprofits Help. These always have the final word.
The free health check scores your account against each rule above, using your own numbers.