Working Effectively with Legacy Code, Michael Feathers. Prentice Hall, 2004.
Michael has distilled a lot of knowledge about how to safely improve code when you lack the safety net of tests. For example, there are places where you’ll take smaller steps or compile more often to be sure that no new problems are introduced. The book is structured with each chapter focused around a major problem you might have. Odds are, you either need this book or know someone who does. (Reviewed Dec., ’04)