Pendragon series(well, part of it)
It's a good series
First book:The merchant of death
The first book is the book that explains almost everything about being a traveler. When you first start the book, it would start as a note to someone called Mark that would be introduced in a few chapters later. Almost the whole book is about the main character Bobby Pendragon going through an adventure in a territory called Dendrun, he tries to stop a civil war from happening in that territory. It also has his two friends Mark, and Courtney read Bobby's journals and they try and find out what happen to Bobby. The first book is recommended to read first since when you jump to book number two, or three or four ,etc... You won't understand what's going on.
Second book:The lost city of faar
Right after the first book Booby knows who is the bad guy and he is trying to destroy all of the territory's. Bobby goes and follows him to the territory of Cloral, with his uncle press. When they follow them they would appear in a cave in the ocean, and his Uncle press would try and tell Bobby about the territory of Cloral and teach him somethings about the territory like how it works. He would meet his uncles friend from the territory of Cloral named Vo Spader. Three of them try and find a way to stop the bad guy and they would find a lost city and they find people are still there and that they know what happening that the crops are poisoned and that they know what to do. But it would get destroyed, but Bobby,Press, and Spader are able to save it and they save the city and the territory. In earth, the something happens like what happens in the last book.