Gutenberg did not sign the book. His name appears nowhere in it, there is no colophon and no printer's device, and the attribution rests on documents outside the object.
The most important is a lawsuit. The Helmasperger notarial instrument, drawn up in November 1455, records the suit brought by Johann Fust, who had lent Gutenberg the money for the workshop, over repayment of those loans. The settlement left Fust and Gutenberg's foreman Peter Schoeffer in possession of the equipment, and Fust and Schoeffer went on printing under their own names. The invention's first great commercial success therefore belonged, almost immediately, to someone other than the inventor.
The other document is a letter. In March 1455 Enea Silvio Piccolomini, a papal diplomat who later became Pope Pius II, wrote to a cardinal describing quires of a Bible he had been shown at Frankfurt. He said the lettering was so clean that a man could read it without spectacles, that he had not been able to obtain a copy because the entire edition was already spoken for, and that between 158 and 180 copies were being made. It is the earliest external notice of European printing, and it records the technology arriving already sold out.
The two documents together are why the book has a maker at all.