The most important source of information for any investor considering a mutual fund purchase is the prospectus issued by the fund's managers. It includes valuable information on fees, investment strategies and objectives, fund performance, risks, and buying and selling shares. Legal requirements set by the Securities and Exchange Commission on what must be included in a prospectus and how it should be worded can make it difficult to understand. The Investment Company Institute, the industry's trade organization, has issued a free guide, "Reading the Mutual Fund Prospectus." It points out the 17 most important items investors should consider. To get a copy, write to the Investment Company Institute, 1600 M St., NW, Washington, D.C., 20036.