When I encounter a sylistic conundrum, I often first refer to Alan Flusser's Dressing the Man. Heavily adorned with photographs of style giants like Fred Astaire, Clark Gable, and the Prince of Wales, and illustrations from Esquire and Apparel Arts, Flusser's Dressing the Man is the ultimate reference on men's style.
In Dressing the Man, Flusser examines the historical development of tailored men's clothing and sets forth the rules for their proper wear based on those origins. He discusses the art of mixing patterns, considerations of proportion for diffferent body shapes, and appropriate colors for varied complexions.
This book will likely not appeal to the casual dresser. With the exception of one chapter (that seems to have been added as an afterthought) on business casual, Dressing the Man focuses mainly on timeless, traditional conservative male attire.
For any man interested in looking his best, Dressing the Man deserves a prominent spot in his library.