I was a kid during soviet time. Propaganda books, i.e. books about revolution was a minority.

Most of the books were regular children books about how to make friends, how to behave, how to study. There were books about science and fairy tales. Prose and poetry. There eas translation of books from authors from capitalist countries.

The kid books was very cheap like 3-5 kopeck (0.03-0.05 of rouble) with the average salary of abou 35 roubles.

The only problem was that most interesting books, teens books were rarely available in book shoops, but you could borrow it at a library. Libraries were available at every school as well as at district and city level.