I just finished John Piper's new book, When the Darkness Will Not Lift (audiobook). It's about 70 pages long, helpfully short for someone who is weighed down with darkness. You can pick it up for $8, or find it online for free thanks to Desiring God.
This is a very encouraging little book. Piper deals with the issues of melancholy, assurance/doubts, unconfessed sins, Satan, self-absorption, remedies, and dealing with other people who are in darkness (among other things). Oh, and Piper all talks a little about joy. :)
This book helped me realize where I'm struggling through a bit of darkness right now It's remedies are not simplistic and trite but rather practical and Gospel-centered. I will try to keep a few copies of this around to give away, and I think it will become one of the most recommended books for those I pastor. I highly recommend it whether you love Piper's writing (like me) or not.
A few encouraging quotes...
Despair of finding any answer in yourself. I pray that you will cease from all efforts to look inside yourself for the rescue you need. I pray that you will do what only desperate people can do, namely, cast yourself on Christ. (p 21)
If we pour ourselves out for others, God promises to make us like "a watered garden"--that is, we will receive the water we need for refreshment and joy. But even more, we will thus be "a spring of water" that does not fail--for others, for the demanding, exhausting draining ministry of urban self-giving. (p 63)