I've read that 日本人の知らない日本語 translates to: "Japanese (language) that Japanese (people) don't know". But I don't understand how or what the の does in that sentence. If I'm not mistaken 知らない日本語 could ...
If, for example, I wanted to say "I like the book that my sister gave me", would it be 姉がくれた本が好きです? I'm using Genki to study, but they don't seem to have any examples of this particular structure ...