A great laymen book on inflation is "The Inflationary Universe" by Alan Guth, who is one of the inventors of inflation.
Inflation is a framework the was invented to explain the cosmological flatness problem, i.e. to explain why spacetime is so flat, and to explain why there are no magnetic monopoles.
A great discussion can be found here: https://telescoper.wordpress.com/2017/07/13/the-early-stages-of-cosmic-inflation/amp/