I recommend http://www.cyberhymnal.org, which has over 6,400 Christian hymns & Gospel songs from many denominations. You’ll find lyrics, scores, MIDI files, pictures, history, & more.
Worship Leader magazine each month features a new song-writing story for worship songs and modern hymns.
I mainly use books to find older hymn stories. Here are some recommendations.
Then Sings My Soul, Book 1 by Robert J. Morgan (Thomas Nelson, 2003)
Then Sings My Soul, Book 2 by Robert J. Morgan (Thomas Nelson, 2004)
O Worship the King by Tada, MacArthur, Wolgemuth (Crossway, 2000).
Amazing Grace by Kenneth W. Osbeck (Kregel, 1990).
101 Hymn Stories by Kenneth W. Osbeck (Kregel, 1982).
52 Hymn Stories Dramatized by Kenneth W. Osbeck (Kregel, 1992).
The Book of Hymns by Ian Bradley (Testament Books, 1989).
Hymns of Faith and Inspiration by Pamela J. Kennedy (Ideals, 1990).
How Sweet the Sound by George Beverly Shea (Tyndale, 2004).