Excellent book cover that instantly captures attention. As a fan of the sci-fi genre, I was drawn to the thumbnail, which is the most important point you want to cover. A minor drawback is the font, which could be more elegant. Even though the book is sci-fi, the font does not need to be that futuristic. Something like Trajan would work great. A bevel and shadow effect similar to the Game of Thrones covers would give it a more finished look. The "Spontaneously written by" part is interesting, but for a major publication it would be a little strange. Otherwise great work!
I always love pictures that tell a story or convey an atmosphere successfully. This one manages that very well. The colors and character designs used in this picture are excellent and the overall story about the puppet master and children's bones is very interesting. The first minor critique of this piece would be about the somewhat stiff expression of the character holding the swords (the face could be exaggerated a bit more, to better convey struggle/pain). The poses could also use more exaggeration. They remind me of traditional Russian folk art and book illustrations, which is an interesting style, but I think more theatrical, contorted hands, arms and legs would make your pictures far more impressive. Everything else about the painting is fantastic.