Few things are worse than being prepared then being blind-sided by some obscure unwritten rule. It's those unwritten rules that get you every time. You know what I'm talking about - those rules that "everyone" knows about but never talks about because, well, everyone just knows them. Like, you don't spit against the wind, you don't tug on superman's cape, you don't pull the mask off the Lone Ranger, and you never, EVER walk across the pitcher's mound on the way to the dugout.
Which, amazingly enough, brings us to our word of the month: UNWRITTEN LAW. According to Black's Law Dictionary (West), Unwritten Law is a rule, custom, or practice that has not been enacted in the form of a statute or ordinance.
As far as law and legal research are concerned, Unwritten Laws are all over the place. For example, the Supreme Court has ruled that the due process clause of the Fifth and Fourteenth Amendments to the U.S. Constitution protects the right to privacy. Problem is, the word "privacy" isn't even mentioned in the Constitution and, hence, falls under the unwritten law category. For an examination of privacy, might I suggest you take a look at Libel and Privacy (Aspen), Privacy and Data Security Law Deskbook (Aspen), Torts in a Nutshell (West) and Privacy Torts (West).
In commercial law, (specifically, under the Uniform Commercial Code) merchants often resolve their disputes by use of custom, trade usage, and generally accepted practices in a particular industry. Excellent resources as they related to the UCC include Damages under the Uniform Commercial Code (West), Commercial Damages: A Guide to Remedies in Business Litigation (Lexis), and Uniform Commercial Code (West).
Finally, the very nature of common law is comprised of decisions which were never written down and are often based on gut reactions to situations and standards that have evolved over hundreds of years. A few (actually interesting) titles relating to the common law might be Restatement of the Law (ALI), Handbook of Common Law Pleading (West), and Readings on the History and System of the Common Law (West).