In the reverse sentence discussion, users Paulenrique and brmarcio write that using the definite article in Portuguese means that you are referring to specific books. In other words, there is apparently nothing arcane or mysterious going on with this sentence, merely that it appears the translation of this sentence is suboptimal at present.
Paulenrique (MOD) writes:
"dos" = (of) the > that is, you are talking about specific books, which is not the case here.
[...] You shouldn't use "dos" unless you're specifying the books. [...]
- ... de livros velhos.
... of old books.
- ... dos livros velhos.
... of the old books.
Reverse sentence discussion:
Yes, that's it. "Dos livros" here sounds less usual to me, since we tend to say "de livros velhos". But, if you are at a library and then you talk about the smell of the old books in a specific department, you can say "Eu gosto do cheiro dos livros velhos", referring to those books.