REIT's Mutual Funds and ETF's ?

It appears that REIT's offer a good opportunity to diversify an investment
portfolio but I am having difficulty in determining which is a good basic
investment guide for REIT trusts, funds, and ETF's. I have the National
Association of Real Estate Investment Trusts' "Investing In Real Estate
Investment Trusts" Guide but I  hope to find additional information and
something more independent.

Does anyone have any suggestions?