Design Patterns by the Gang of Four the Second (or possibly Third depending upon punk rock precedence) Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides.
Ward Cunningham has a repository of design patterns online.
Given a large sample of code, we name the commonest solution forms "design patterns". The benefit of learning the patterns lies in code maintainability as pattern adherence extends the One Way To Do It principle. Write code for large projects? Gotta know 'em. ;)