You can do this, and it's probably intended as a general statement, consideing that old books were mostly wrapped in leather.
In English you usually formulate general statements with plural subject without an article: "Good children don't play in the streets." While in Hungarian you mostly use singular with an article: "A jó gyerek nem játszik az utcán." There are several other variants how you could form a general statement in ither language, but these are the most used ones.
Now, if we translated these two sentences literally in the respective other language, a difference of meaning would open up. "Good children don't play in the streets." - "Jó gyerekek nem játszanak az utcán." That means that there are good children who don't play in the streets, but it doesn't encompass all good children.
On the other hand: "A jó gyerek nem játszik az utcán." - "The good child doesn't play in the streets." This sounds in English like there is just one good child we're talking about, and again not the entirety of good children.