In a filing made with the Securities & Exchange Commission this morning, William Ackman said he was prepared to back a $16 per share offer by Borders to buy Barnes & Noble. Ackman and his Pershing Square Capital Management firm have held a significant stake in Borders for over a year, but in recent months Ackman has taken a back seat to Bennett LeBow. In the filing however, Ackman disclosed that he has upped his stake in Borders to 37% from 31.5% in the spring. B&N had no comment on the report. In an interview with PW before Ackman’s bid was disclosed, B&N chairman Len Riggio—PW’s Person of the Year—said he hoped the strategic review being conducted by B&N would be completed in the first quarter of 2011.

According to the filing, Ackman is prepared to make an all-cash transaction for B&N's shares or a mix of stock and cash, if B&N stockholders "prefer to share in the substantial synergies of the business combination and receive equity in the combined company." The filing did not indicate what repsonse, if any, it received from B&N.

Borders, still struggling to return to profitability, will release its third quarter results after the market closes on Thursday.