Currently reading, and loving, this biography of Robert Murray McCheyne.
Here's the story behind the cover:
'The rusty key symbolises some aspects of the security found in Christ's love. The key was allowed to rust because, once it was used to lock securely, it was never required again.
In the same way the believer is secure in Christ's love, Christ does not let the beloved slip from His grasp, but holds them firmly throughout eternity.'