(MONEY Magazine) – Would you believe it? Genealogy has become the third biggest hobby in America, after collecting coins and stamps. If you want to have fun tracing your family tree, however, watch out for cheesy firms that offer to mail you a book with the names of all your relatives through history for $25 to $50. ''It's unlikely such a book will be any help,'' says Carolyn Nell, president of the National Genealogical Society (NGS). ''Many are just names and addresses of living persons with your surname, culled from public phone books.'' For genuine help, get the new free pamphlet Branching Out: Tracing Your Family Tree (800-950-4873), published by NGS and MCI, the phone company known for its ''friends and family'' ad campaigns. The booklet offers six pages about tracking your ancestors and the names of five reputable genealogy sources. Before getting serious and asking one of the 3,000 legitimate genealogical researchers for help, bear in mind that costs can ratchet up pretty quickly. Those people, most of whom are certified by the Board for Certification of Genealogists, charge $15 to $35 an hour. It's best to have a genealogist work a set number of hours, say 20, and then review his or her progress before getting more deeply involved.