It said that Mrs Bush, after consulting with relatives and doctors, had made a decision to "focus on comfort care" at the family home in Houston.
Eight years after she and her husband left the White House, she stood with her husband as their son George W. was sworn in as president.
"They are surrounded by family, but I think the fact that they're together in this and he still says, 'I love you Barbie, ' every night is pretty remarkable", she said. Proceeds from the books benefited adult and family literacy programs. The other was Abigail Adams, wife of John Adams, the nation's second president, and mother of John Quincy Adams, the sixth president. The couple had six children.
Both Barbara Bush and her husband, who live in Texas, have confronted numerous medical difficulties in recent years. Bush told an interviewer, "I sat next to her once, thought she was lovely, and I think she's very happy in Alaska, and I hope she'll stay there".
The statement provided no details on her condition, but Mrs Bush had been hospitalized for bronchitis treatment in January 2017.
Bush Hager, a correspondent for NBC's Today show, told viewers that she spoke to her 92-year-old grandmother on Sunday night after the announcement about her health.
The statement didn't elaborate on the former first lady's health problems, but she has been suffering from congestive heart failure, according to reports.
She and her sister, Barbara, are the daughters of former President George W. Bush.
He was treated months earlier, also for pneumonia.