The General history of the things of New Spain, written by Fray Bernardino de Sahagún, was the result of a long compilation process carried out between 1547 and 1585, initiated shortly after the conquest of Mexico by the Spaniards. It is also known as the Florentine Codex, since a version is in the Medicea Laurenziana Library of Florence, Italy. The complete codex can be consulted online at the World Digital Library (WDL).
The online edition linked here is the History of the conquest of New Spain (Book XII of General history) from the Early Nahuatl Library, Stephanie Wood, editor, with many contributors, especially James Lockhart.
It contains the illustrations of the Book XII of Florentine Codex and the texts in Spanish and Náhuatl, with the Spanish text translated into English, and the Náhuatl text translated also into English.