With interest rates falling, the author of William E. Donoghue's Complete Money Market Guide (1981) gives his undivided attention to open-end no load companies, better known as mutual funds. There are funds for virtually any purpose--from aggressive growth through tax-exempt income--and Donoghue examines most of the prototypes (in particular, equity-oriented vehicles) in terms of individual goals. To those who want to stay ahead of developments, he recommends a ""12 percent solution."" This involves monitoring the average maturity index of MMF (reported weekly in major papers and financial journals)--which moves, says Donoghue, counter to the trend of future interest rates. When it goes up (as it has recently), the cost of money can be expected to decline (and vice versa). Money should be moved at around the 12 percent level. If a change is imminent and MMF yields drop near or below 12 percent, it's time to shift one's assets into a stock fund--preferably, an affiliate within a group that permits no-charge switching. When MMF yields rise to 12 percent again, it's time to scramble back. Donoghue is not dogmatic about his brainchild, however, and advises SLYC--safety, liquidity, yield, catastrophe-proofing--strategy as well. The greatest risk in using the system, he cautions, may be the temptation to depart from the do-nothing policy that's indicated most of the time. Included are pointers on selecting and purchasing no-load funds, and on how they can fit into IRAs or other tax-favored pension plans. Some huckstering too (for Donoghue's advisory services)--but extensive and up-to-date as a survey of the mutual funds industry.