St John of God is patron saint of booksellers and indeed of the entire book trade; also of hospitals and the sick.
João Duarte Cidade was born in Montemor-o-Novo, in southern Portugal in 1495, but was abducted or lost at the age of eight. He was taken in as an apparent orphan and put to shepherding. When he was about 22, seemingly to escape the attempts of his boss to make him marry his daughter, he ran away and became a military man, fighting as a mercenary for Spain against the French and Turks in Hungary. He spent the next 18 years as a mercenary, and when his troop was disbanded went to Andalusia and became a shepherd once more. When he was about forty he was converted by hearing a sermon preached by St John of Avila, and aimed at a martyrdom in North Africa helping Christian slaves. He was talked out of this plan and became a book pedlar in Gibraltar, with such success that he opened a bookstore in Granada in 1538. He soon appears to have suffered a breakdown, running through the streets rending his hair and giving away his book inventory. He got out of hospital in 1539 and filled his house with poor sick people. His final illness was incurred while rescuing a drowning man from a flood, and he died in 1550 at the age of fifty-five before the altar of his hospital’s chapel. His followers formed the Brothers Hospitallers, naming him as their founder.
I guess it’s obvious why he became the Parton saint of the bookselling. But who gets to appoint patron saints? One can see how in medieval guilds a patron saint might be adopted. It happens that Saint John the Evangelist is apparently patron saint of editors and authors. Saint Augustine of Hippo is patron saint of printers. Saints Genesius and John of God get a look in in the printing industry too, while St Brigid of Ireland is patron saint of printing presses. Pope Celestine V is patron saint of bookbinders. He’s referred to as Pope not Saint Celestine although he was pope for five months only (he resigned), and has been a saint for over 700 years. The ways of the lord are indeed mysterious. John of God gets the clean-up status, looking out for the book trade in general.