This is the 213th most frequent Spanish word.
buen
"Buen" is the shortened form of "bueno," meaning "good," used before masculine singular nouns.
'Buen' is an adjective, used before a masculine singular noun to describe a positive attribute about the person or object.
Es un buen amigo.
He is a good friend.
'Buen' is used here to modify a masculine singular noun, emphasizing a favorable or pleasant characteristic of the day.
Hoy hace un buen día para pasear.
Today is a good day for a walk.
'Buen' again functions as an adjective to indicate the quality of the noun it precedes, implying the book is of high quality or interest.