The Researcher's Guide to American Genealogy by Val D. Greenwood is a classic genealogy text. First published in 1973, you'll still find The Researcher's Guide being used as a textbook in many genealogy courses.
Published in 2000, the third edition can feel old-fashioned. The sections on correspondence and computers are especially outdated. But chapters on evidence and standards of proof provide excellent introductions to these topics for the casual researcher. Greenwood also offers timeless information on genealogy subjects such as records, in-person research, terminology, and organization.
An essential text for the genealogist wishing to become a more serious researcher.