Some excerpts from the Unesco site:
By celebrating this Day throughout the world, UNESCO seeks to promote reading, publishing and the protection of intellectual property through copyright.
23 April: a symbolic date for world literature for on this date and in the same year of 1616, Cervantes, Shakespeare and Inca Garcilaso de la Vega all died. It is also the date of birth or death of other prominent authors such as Maurice Druon, K.Laxness, Vladimir Nabokov, Josep Pla and Manuel Mejía Vallejo. It was a natural choice for UNESCO's General Conference to pay a world-wide tribute to books and authors on this date, encouraging everyone, and in particular young people, to discover the pleasure of reading and gain a renewed respect for the irreplaceable contributions of those who have furthered the social and cultural progress of humanity.
The idea for this celebration originated in
Happy World Book Day! Books rock. Books are the windows and doors to the world!