Berkshire Annual Report and Shareholder letter.

Berkshire annual and quarterly reports.

Link to all Buffett's letters to shareholders

A couple of interesting excerpts:

Our direct currency positions have yielded $2.3 billion of pre-tax profits over the past five years,
and in addition we have profited by holding bonds of U.S. companies that are denominated in other
currencies. For example, in 2001 and 2002 we purchased €310 million Amazon.com, Inc. 6 7/8 of 2010 at
57% of par. At the time, Amazon bonds were priced as “junk” credits, though they were anything but.
(Yes, Virginia, you can occasionally find markets that are ridiculously inefficient – or at least you can find
them anywhere except at the finance departments of some leading business schools.)
The Euro denomination of the Amazon bonds was a further, and important, attraction for us. The
Euro was at 95¢ when we bought in 2002. Therefore, our cost in dollars came to only $169 million. Now
the bonds sell at 102% of par and the Euro is worth $1.47. In 2005 and 2006 some of our bonds were
called and we received $253 million for them. Our remaining bonds were valued at $162 million at
yearend. Of our $246 million of realized and unrealized gain, about $118 million is attributable to the fall
in the dollar. Currencies do matter.

and

We made one large sale last year. In 2002 and 2003 Berkshire bought 1.3% of PetroChina for
$488 million, a price that valued the entire business at about $37 billion. Charlie and I then felt that the
company was worth about $100 billion. By 2007, two factors had materially increased its value: the price
of oil had climbed significantly, and PetroChina’s management had done a great job in building oil and gas
reserves. In the second half of last year, the market value of the company rose to $275 billion, about what
we thought it was worth compared to other giant oil companies. So we sold our holdings for $4 billion.

Warren Buffett is known as a long term buy and hold investor but he has been a great bond trader over the years.