AceShowbiz - Justin Bieber has turned to a top self-help book for marriage advice as he prepares to wed model Hailey Baldwin.

The Sorry hitmaker proposed to Hailey while on vacation in the Bahamas in early July, and it seems he has turned to American theologian Timothy Keller for some words of wisdom to help him prepare for the relationship journey he is about to embark on.

Bieber was spotted leaving his fiancee's New York City apartment on Wednesday night, August 08, carrying a copy of Keller's 2011 release, The Meaning of Marriage: Facing the Complexities of Commitment with the Wisdom of God.

He also suggested he's already been putting some of the lessons learned into practice, after he was photographed breaking down in tears with Hailey during a bike ride in the Big Apple on Tuesday.

Asked about the emotional incident on Wednesday, Bieber held up the book to the TMZ camera and explained, "You got good days and you got bad days. It's not real if it (relationship) doesn't have any bad days."

Justin, 24, has been embracing Christianity in recent years after overcoming a period of bad boy behaviour, and he was baptised in 2014, when he sought guidance from Pastor Carl Lentz, a senior preacher at New York's Hillsong Church.

He has continued to attend services ever since and recently took Hailey to the Hillsong Church conference in Brooklyn.

The model, 21, is also religious - she was raised as an evangelical Christian, and her actor father, Stephen Baldwin, is a born-again Christian.

The young couple previously dated in early 2016, but rekindled its romance in June, three months after Justin split from his on/off girlfriend Selena Gomez.