Yet another suggestion: В стране невыученных уроков ("In the Land of Unfinished Homework"), by Liya Geraskina. A 1965 children's novel with a not-too-completed plot, and lots of natural, idiomatic dialogue. Somewhat similar to Norton Juster's The Phantom Tollbooth (boy who slacks at school visits a magical country where words and numbers come to life) but without all of the elaborate Carrollish wordplay of Juster's book -- which of course makes it easier for you as a Russian learner.

Once again, the full text is available on lib.ru, but there's no standard English translation to help you in this case. On the other hand, the 20-minute cartoon version by Союзмультфильм became an instant classic in its own right, and you can easily find it on YouTube to give you the basic outline of the plot.