The devil is definitely 110% real. Some saints (such as Padre Pio) even battled physically with the devil. And he is not the cute little guy with the pitchfork and the tail. One book I would suggest is Screwtape Letters by C.S. Lewis. It is about Screwtape (a senior devil) sending letters to his nephew (Wormwood) giving him advice on how to draw away his "patient" from God. Another good book which gives a good depiction of the devil is the Forty Dreams of St. John Bosco: The Apostle of Youth (this is an excellent book). With God on our side we can defeat the devil and prevail against evil.
VII "BUT DELIVER US FROM EVIL" (CCC #2851) In this petition, evil is not an abstraction, but refers to a person, Satan, the Evil One, the angel who opposes God. The devil (dia-bolos) is the one who "throws himself across" God's plan and his work of salvation accomplished in Christ.