Years of statistical research and number-crunching have proven one thing: Earl Weaver was a pretty smart guy. Weaver, who didn't frown on using numbers himself, was a living hybrid of old-fashioned baseball wisdom and a forward-thinking organizational mind that helped turn the Baltimore Orioles into a dynasty.
For the fan, you can read about how Weaver achieved such great success in the majors with methods not out of line with modern, or "sabermetric" analysis.
For a manager or coach, Weaver offers a lot of practical advice about managing a ballclub. This won't teach you how to hit a curveball, but it will tell you what lineup to use against a curveball pitcher, how to handle players both young and old, and how to run an effective spring training regimen.
Weaver's advice is pertinent and insightful even now, 20 years after he managed his last game.