My portfolio is fairly diversified. It's not that I don't understand the theory behind concentrated investing, I just don't have the confidence that I can tell the difference between a great and a good idea, and so I spread the eggs around. But I hope to get there someday. In The Focused Few, Richard Rockwood offers a few ideas on getting there.
Rockwood takes a close look at the principles of the most successful value practitioners. As anyone who follows guys like Buffett, Munger, Klarman etc know, these guys have offered a lot of wisdom to the public. Rockwood, clearly an avid reader, has accumulated a great deal of wisdom following these guys, and shares some of it in this book.
For experienced value investors, the book may be a bit redundant and repetitive. But for beginners, I'd say this is as good a book as any with which to begin understanding how to think about the stock market.