In trading on Monday, shares of American Capital Agency Corp (NASD: AGNC) crossed below their last reported book value -- defined as common shareholder equity per share -- of $28.93, changing hands as low as $28.90 per share. American Capital Agency Corp shares are currently trading down about 2.3% on the day. The chart below shows the one year performance of AGNC shares, versus its 50 and 200 day moving averages: