That's the question I like best. I like it when people ask me, and I like to ask it of others. Not that anyone has asked, but here's my answer:
Sherwood Wirt's Jesus, Man of Joy. I highly recommend it. This book made me aware that I've been short-changing the second fruit of the Spirit, joy. When joy is absent, the kingdom we preach is but a pale and wavering mirage. Let me just say that this book changed me some. I don't think I'll ever forget it.
If you saw The End of the Spear, you may have been prompted to find a book about the events dramatized in that movie. I know I was. So I made use of my local public library and came away with Jungle Pilot, by Russell T. Hitt. A fine book, this. It's a biography of Nate Saint, who was the central character in the movie, of course. A remarkable man, he reminds me in many ways of my own father. Not that that should mean much to you, but it did to me.
Of course the best-known book about those events is Elizabeth Elliot's Through Gates of Splendor. I've just started it. Here's a paragraph from Jim Elliot's journal, quoted early in the book:
'He makes his ministers a flame of fire,' Am I ignitable? God deliver me from the dread asbestos of 'other things.' Saturate me with the oil of the Spirit that I may be a flame. But flame is transient, often short-lived. Canst thou bare this, my soul--short life? In me there dwells the Spirit of the Great Short-Lived, whose zeal for God's house consumed Him. 'Make me Thy Fuel, Flame of God.'