The first appearance of a "true musical idea" in a printed book was on a page of Grammatica by Franciscus Niger (Francesco Negri) printed in Venice by Theodorus Herbipolensis, Francus, for Johannes Lucilius Santritter on March 21, 1480.
"It occurs in that section of the grammar which deals with the rhythms of five metres used by various Latin poets, and shows how the verses were chanted in schools. This passage is doubly important as containing the earliest known printing both of secular music and of mensural notes. There is little doubt that the notes were printed from type and not, as some authorities believe, from a metal block. The staves were to be added by hand. In the second edition of the same work (Basle, c. 1485) the same notes are printed from a wood-block, which is thus the first used for secular music" (King, Four Hundred Years of Music Printing  11).
In October 2012 a digital facsimile was available from the Bayerische Staatsbibliothek, München at this link.