Times Mirror Unit Buys Publisher: Mosby-Year Book, a unit of Times Mirror Co., said it has acquired Great Performance, a publisher of educational materials to promote consumer and employee health, for an undisclosed price. Mosby-Year Book, based in St. Louis, is a leading publisher in health science. Great Performance, based in Portland, Ore., has 48 employees and posted 1993 revenue of about $5 million. Its name will be changed to Mosby-Great Performance, and Mark J. Tager, a physician who founded Great Performance, will remain as president. Los Angeles-based Times Mirror is a diversified publishing and information company that publishes the Los Angeles Times, Newsday and New York Newsday newspapers.