Billionaire Warren Buffett wants Americans to be optimistic about the country's future. But also to be wary about borrowing money and the games public companies play with profit numbers they report.

Buffett said in his annual letter to Berkshire Hathaway shareholders that he still believes America's best days are ahead. He said a housing recovery will likely begin within the next year, which would help the economy and several Berkshire subsidiaries.

Buffett's letter detailed how the acquisition of Burlington Northern Santa Fe railroad, better results at Berkshire's other subsidiaries and a 1.9 billion US dollar paper gain on investments and derivatives combined to boost the company's net income by 61 percent. The letter was full of good news for Berkshire investors because nearly all of its businesses, except the ones linked to housing, performed well.