What is the theme of the book Left Behind?

The theme is the end of The Earth with The Rapture and The Tribulation, and The Coming of Jesus Christ. As the title says, it deals with those left behind by the "Rapture".

The Rapture and Tribulation are concepts devised by John Nelson Darby, a British evangelical preacher and founder of the Plymouth Brethren, in 1830. He said that believers would be "raptured" up into heaven and that this event would then be followed by a period of Tribulation of non-believers. In Left Behind, we also see the antiChrist, a concept that has popped up in different contexts over the centuries and is always bound to arouse interest.

In the Bible, Jesus will come once, at the end of the world, but in Rapture theology he will come twice - once to "rapture" his church bodily up into heaven, and again to fight the armaggedon and found his thousad-year kingdom on earth.