The Jesus Calling application is available for a one-time fee on iTunes and Google Play, as of June 2015. The application is based on a book by Sarah Young, available on Amazon.com.
Sarah Young wrote "Jesus Calling: Enjoying Peace in His Presence." It's also available as on e-book on the Amazon Kindle. The book and application feature scenarios with life problems and answers Young provides based on the Bible.
Young kept a journal of her prayers for years, then began focusing on what she felt Jesus was telling her. The book has prayers and devotions by Young, along with highlights of the Bible.