If you're a foreigner looking for an easy Turkish book series to improve your language skills, but you don't want to read some stupid children's morality tales, GET THIS BOOK.
Briefly, it's about two kids whose parents disappear on an archaeological dig. They're sent from their boarding school in Venice to live with their only relative, an uncle they've never met who runs a hotel in Istanbul. On the way they run into a villain, a series of clues, and a magical mystery that seems to have been secretly pursuing them their entire lives. It's essentially a mystery series, with a light dusting of magic and plenty of humor as well.
The series is clearly influenced by Lemony Snicket's classic A Series of Unfortunate Events. Aside from the orphaned(?) kids, it's got a sly cleverness and a darkness to it - an almost gothic tone - that reminded me constantly of ASOUE, which is really incredibly high praise. But whereas the City that ASOUE takes place in is left deliberately vague, here the city of Istanbul is a character in its own right, with all its grand, glamorous, mystical and bloody history.
I'd happily read this series even if it were in English, but when you're trying to push through a book in your second language, an engaging story that keeps you guessing, laughing and wanting to turn the next page makes a world of difference. The first part of the book may seem to drag a bit, but stick with it past the exposition. As soon as the kids get on the train to Istanbul, the narrative picks up and never stops after that.
The language is easy enough for a lower-intermediate reader, although some of the magic- and history-specific vocabulary will take some getting used to. There's also quite a bit of humorous wordplay, which is fun and rewarding to puzzle out.