Much of the earliest written music that still exists today was for plainchant. With time it became more common to record secular music as well as sacred. The chansonniers of the troubadours are important examples of period secular music. As it became more prestigious to own books with ornate illumination written music started becoming works of art.
With the advent of the Gutenberg press it was only a short time before music started to be printed. Part books were often printed in sets of four to six, where each book contained the same songs but for a different vocal part.
Restrictive licensing in England meant that printing there was almost exclusively done by William Byrd. However in 1597 lute scores in movable type started to be printed, and these form the basis of most of what we know about the so-called English lute school.
Period sheet music is, as one would expect, now in the public domain (though modern edited versions of songs may not be). There are several projects that aim to archive public domain sheet music, one of which is Mutopia.