My 9 yo son loves the Warrior series by Erin Hunter. After the first six there is another Warrior series (The New Prophesy) that he's just starting.

My 11 yo son loved "The Takers"- the first in a trilogy by R.W. Ridley. He doesn't read a lot, so when he devoured that I had to run out and get the second one: "Delon City". He was reading it today in the carpool.

My 13 yo reads everything--can't keep track. Currently he's reading "Cabin Pressure" by Josh Walk.