This is the 2419th most frequent Spanish word.
libro
Book.
Here, 'libro' is used as a noun referring to a physical object, a book, described by the adjective 'rojo' (red).
El libro rojo está sobre la mesa.
The red book is on the table.
In this sentence, 'libro' functions as a noun that denotes the object being read, qualified by the adjective 'interesante' (interesting).
Estoy leyendo un libro interesante.
I am reading an interesting book.
'Libro' serves as the direct object of the verb 'firmó' (signed), specifying what was signed by the subject, the author.