"the key that unlocked practical printing was the devising of movable metal type, cast in molds producing strong and uniform letters and producing them with relatively good speed." (chappell, page 3)up until this time, relief printing, raised surfaces against paper, the only printing being done in europe was wood block printing.
the advent of popular printing was a convergence of many forces that took years to bear fruition:
and interest level - most folks just couldn't read and didn't care, until enough folks in the courts of the king, and by extension the burgeoise had to learn literacy and so took it home, and the desire for more home-suited reading, perhaps for pleasure.
the first books were designed to compete directly with calligraphers (chappell, page 12
guttenberg was a goldsmith, well suited for cutting the metal molds
"...from 1450 or thereabouts until 1470, fourteen cities could boast printing offices. From 1470 to 1480, the number had grown to more than one hundred." chap page 72 point system for type size - page 50