My strong opinions against islam aside, I find there is nothing wrong about reading the opponent or anyone else's book, especially if it is just to understand the way they tick better. How can we reach out to them if we don't even know their core beliefs?
It's a question of time spent on it. Learning what others believe so you can answer their questions is fine but make your top priority learning what you believe. Grow in faith and understanding of God lest you be sucked into the deceptions other religions will expose you too or have no answer. Fighting without the armor of God is folly (Ephesians 6:10-18).