Mary Beth, thanks for sharing this excellent resource. And thanks 'rosall' for asking the question.
Although there are many intangible benefits of libraries, funders definitely like to see some facts and figures. There is an equivalent message in this [url http://webjunction.org/do/DisplayContent?id=8496 ]Dear Mayor letter[/url] written by Bob Watson, a library director in Illinois. You might also want to look at the [url http://webjunction.org/do/DisplayContent?id=1204 ]Demostrating Impact: Making Your Case[/url] focus page.
btw, the PowerPoint above is a big file (>7 M

and takes awhile to download, even on a fast connection. However, it is worth it --it's beautifully put together and is packed with great information and easy-to-read charts.