Is the book Lilies of the Field a true story?

No, but.... There is a loose updated Tie-In with, Maybe- The Agony and the Ectasy which was about the painting and construction of the Sistine Chapel ( By Michaelangelo) in Lilies the itinerant Black handyman is more or less cajoled into the constuction of a Convent Chapel by the Nuns. ( P.S. the Sistine Chapel is NOT a convent- it is named in honor of Pope Sixtus) there are very loose connections of a sort of hustled charity... again but but but.
It was based on a novel, okay. helped popularize the song (Amen) by the Impressions. I like the parody version about Anastasia- which has the closing line- But she snuck out the Back Door!- like the episode in the Amy Irving movie when she is in hot water with the KGB.