Sergei Lukyanenko’s works.

I read "Knights of the Forty Islands," which some recommend as children's fantasy.

"Knights..." is an imitation of my favorite writer, Vladislav Krapivin. And, I must say, it's not a very good imitation :( Frankly, let Lukyanenko write his own things; he didn't quite succeed in writing for children. As a standalone book, it might not be bad if you don't know the origins.
Overall, the book is a 3+, although it certainly makes you think.