I watched this show some ~7 years ago - I remember it aired approximately the same time as Smallville, so let's say 2000s to be sure - in Russia (but the series certainly wasn't Russian-made). It seemed fairly recent (at that time).
The main "feature" of the show was a circular stone built in a wall. Two teens (boy and girl) from different cities (possibly even different countries) discover that turning, or otherwise manipulating the stone could be used to open a portal to another city - in fact, this is how they meet, if memory serves.
Throughout the series the stone appears to be limited and malfunctions regularly - at some point the insert something like a knife between the stone and the wall to make it work.
There was a book somehow connected to the stone - a sort of manual. They couldn't use it because it was damaged for some reason, so they give it to a friend, who claims he has some expertise in binding/restoring books after attending some workshop.
At the end of the series this friend finishes restoring the book, and uses the portal to go somewhere, leaving a note along the lines of
I know your secret now. The stone and portal are somehow connected to meteors.
The show didn't seem overtly juvenile-oriented, though being a child myself at that time, I'm probably remembering this wrong :). I feel like this is a fairly popular and famous show, but googling related terms only yields irrelevant stuff.
