Stephanus Byzantinus' gazetteer, De urbibus -- also known as the Ethnica. This is an editio princeps (first ever printing) of this geographic work on cities in the ancient world. The text is from the 5th century, but the incomplete manuscript Aldus used is from the 6th century. It is notably missing the section for the letter 'F'. In the Preface, Aldus encourages readers to add the missing text, should it be rediscovered.
View the pages of the 1502 Stephanus Byzantius.