Merriman is editor and publisher of The Fund Exchange; Dowd is a syndicated financial columnist. Here they make a case for the long term advantage of repeatedly purchasing mutual funds at periodic low prices and selling atguess whathigher prices. The key to identifying ideal times to take action, they say, is to track moving averages of fund prices. For individuals disinclined to such statistical rigors, Merriman reports on several market timing services, including his own. Notwithstanding some inaccuracies caused by recent changes to the tax law, the detailed explanation of the concept and basic technique of market timing makes this an acceptable purchase for most libraries. Joseph Barth, U.S. Military Academy Lib., West Point, N.Y.