Warren Buffett’s letters to the shareholders of Berkshire Hathaway are read and quoted widely, coming to be regarded as one of the most important annual publications in the business and investing world.
Over the years the letters have covered, in quite some detail, almost every important aspect of investing. The (incredibly lacklustre) Berkshire Hathaway website provides an archive of these letters (back to the 1970s, at least) and many other documents also worth a browse.