The Hidden Smile of God
The Hidden Smile of God: The Fruit of Affliction in the Lives of John Bunyan, William Cowper, and David Brainerd by John Piper is an excellent book which is 170 (standard-sized) pages long, which details the way Bunyan, Cowper and Brainerd were comforted and strengthened through trials (from imprisonment, depression and physical illness, respectively) by understanding that God loved them and was sovereign over their lives and painful trials. It is one of four books in Pipers excellent series, The Swans Are Not Silent.