Financial 'survival' for the (almost) rich - a new guide; How to Survive on $50,000 to $150,000 a Year, by Stanley J. Cohen and Robert Wool. Boston: Houghton Mifflin Company. 246 pp. $13.95.

BOOKS on financial planning share a common problem: There are simply too many of them. The challenge for someone writing another volume on this subject is to find some way to make it look different. Mr. Cohen, a certified financial planner and senior vice-president of a New York brokerage house, chose to target his book at an income segment that is not making enough money to be considered truly ''wealthy'' (by some, at least), but is pulling down enough to be looking for help. When all is written and done, however, there isn't much that separates this book from all the others, though it does a good job of pointing out the need for tax planning.

Beginning this year, single people with taxable incomes - not gross incomes - of at least $81,800 and married couples with taxable incomes of $162,400 and above are in the 50 percent bracket. So half of anything they earn, if counted as ordinary income, goes to the federal government. Thus, while a stock portfolio earning 20 percent a year sounds excellent, that return becomes 10 percent after taxes, and 6 percent if the erosion of buying power by a 4 percent inflation rate is counted. So it is important to see the effect of tax-saving ideas like buying municipal bonds or bond trusts, shifting assets to children, or building tax-deferred retirement savings.

Beyond the tax planning methods, the book is a good, though more or less standard, primer on investing in stocks, bonds, real estate, and tax shelters. Cohen runs into problems with mutual funds, though. His lengthy defense of ''load'' funds, where the investor pays as much as 8.5 percent in charges, is curious - until you realize that as a broker, Cohen himself receives part of that fee, something he does not point out. He says that no-load funds normally are not diversified enough, and he wants a company with at least four funds. There are, however, several companies with more than a half-dozen funds that permit easy switching, usually by phone.

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