All of these are excellent suggestions. I really love Terry. He was a great writer and will be missed.
I'll throw my own suggestion in the mix: Treasure Island by Robert Louis Stevenson. It's considered a YA (Young Audience or Young Adult, depending on who you ask) book, but it's a classic English tale about a boy seeking lost treasure. I don't think I need to say more. :) If you've ever heard anyone say "Yo ho ho and a bottle of rum!" this is the novel they are quoting. :D
It's a very easy read too, so if you're still learning English it's a very good option.