There is some material on the IRS.
http://www.irs.gov/charities/charitable/article/0,,id=123204,00.html
http://www.irs.gov/pub/irs-pdf/p561.pdf
http://www.irs.gov/pub/irs-pdf/p526.pdf
http://www.irs.gov/pub/irs-tege/pub4302.pdf
Other sites.
http://www.boardsource.org/Knowledge.asp?ID=3.135
http://www.nonprofitlaw.com/fund/gift.shtml
http://nplawcast.com/2008/10/nonprofit-law-podcast-46-charitable-receipts/