Chancellor and Mears share trade secrets; The News Business, by John Chancellor and Walter R. Mears. New York: Harper & Row. 181 pp. $12.95.

When I started reading this book, I thought it was yet another volume for my shelf-load of practical guides for the aspiring journalist. But it's much more. John Chancellor, the commentator on NBC's ''Nightly News,'' and Walter R. Mears, the Pulitzer Prize-winning Washington bureau chief of the Associated Press, take the reader beyond the newsprint and television cameras to show how news is gathered and molded under the pressure of daily deadlines. They explain the fundamentals, such as how they pick the opening line of a story and infuse their ''copy'' with color and analysis.

In addition, the authors offer personal insights and experiences which illustrate their points. This is done through occasional passages in which each author speaks in his own voice. It makes for fascinating reading, particularly for anyone curious about how events make their way into the media.

You learn, for example, the special problem created when the American hostages in Iran where released on the same day as President Reagan's inauguration. Mears tells of the struggle he had cooking up a single opening sentence for a story that combined both events. Chancellor, meanwhile, had to decide which would get the opening slot on the ''Nightly News'' program. (Mr. Reagan, shown taking the oath, got top billing, followed by a full news report on the hostages.)

The book also has plenty of practical tips for budding journalists. One chapter, for instance, gives ideas on how to get started in the business, while another emphasizes how words are used by newswriters. The latter includes an admonition against useless modifiers, such as saying a candidate ''successfully captured'' the nomination. We're reminded that anyone who captures the nomination must have been successful.

The book is also packed with bits and pieces of newsroom folklore. For instance, until the 1940s, Time magazine had an unwritten rule that whenever New York Mayor Fiorello La Guardia was mentioned, he had to be described as shaped like a fireplug. The point: colorful descriptions should only be used if they make a story more understandable to the reader.