“The Art of Readable Code,” by Dustin Boswell and Trevor Foucher, O’Reilly Media; 1st edition (November 23, 2011), 978-0596802295.
This book is packed with simple rules that will make your code easier to read and understand. Most of them are common sense (well, if you put some thoughts into it when writing code), but still a good refresher. The book is summarized by the Pedro Morais and by Baidu in these sets of slides. (Estimated time to read the book: 1 day)