I am reading The Screwtape Letters right now. For those that don't know, this book is by C.S. Lewis and is framed as a series of letters from one demon to his nephew, an apprentice demon. It is, thus far, an interesting view into human nature and temptation.
If you have ever been through a prolonged period of unfortunate circumstances, you will probably find this quote poignant and encouraging:
[The devil's] cause is never more in danger than when a human, no longer desiring, but still intending, to do [God's] will, looks round upon a universe from which every trace of Him seems to have vanished, and asks why he has been forsaken, and still obeys.