* Libros rojos: This is the most literal translation, simply meaning "red books." This could refer to any books with red covers.
* Libros rojos de contabilidad: This means "red accounting books." Historically, and sometimes still, accounting ledgers were bound in red.
* El Libro Rojo (referring to a specific book): If you're thinking of a particular book known as "The Red Book," you'd need to specify which one. There are many books with this title. For example, Carl Jung's *Liber Novus* (also known as the Red Book) would be *El Libro Rojo* in Spanish.
* Libros con tapas rojas: This means "books with red covers," a more descriptive phrase.
To get a more precise answer, please provide more context. What kind of books are you thinking of? Are you looking for books about a specific topic that *happen* to have red covers, or is the "red" aspect crucial to the meaning?