A statement from the office of her husband, former president George HW Bush, described her as a "relentless proponent of family literacy".
Mrs Bush, the only woman to have seen her husband and son sworn in as US president, had been unwell for some time and had declined further medical treatment.
She said she wanted to spend her last days in "comfort care" with her family.
Her husband, at 93, is the longest-lived US president.
Their son, George W Bush, was elected president in 2000 and served two terms as the nation's 43rd president.
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