This Present Darkness by Frank Peretti - it's really old now, but am presently rereading it. It has so helped me in my christian life seeing things from a different perspective. About a small town coming under spiritual attack with demons and angels fighting them, and how prayer changes things.
Now be open minded about my next recommendation ( ;)

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A voice in the wind" by Francine Rivers.
Now she may usually be more of a writer for females (?). But this book because it is quite gruesome talking about gladiators in the lion's den, and sexual disease etc its not the usual kind of book. This to me is the best book I have read. It so helped me, seeing the sin and depravation in Rome in biblical times, made me understand the background of the new testament, and is an excellent author.
Now she has 2 sequels to it as well (didn't like the 3rd one)
There are many more but those two books are a MUST read for everybody (IMO

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