Ancient rock art

I found these at my family's summer place. What are they? They're painted rocks that are supposed to look like - well, books, of course! The green one says "Satuja" (Fairy tales) and the red one is "Hamlet". I can't quite remember when I made these, but it must have been over 20 years ago! I do remember elaborate plans to make many more (all my favourite books, of course), but if I did make any, I can no longer find them. Perhaps it was too much of a challenge to find rocks of a suitable shape. Still, I like the idea, and it was interesting to see that my love for books was such a big influence all those years ago already.