Language and a sign of the zodiac are in these books (12)
I believe the answer is:
dictionaries
'these books' is the definition.
(dictionary is a kind of book)
'language and a sign of the zodiac' is the wordplay.
'language' becomes 'diction' (I am not sure about this - if you are sure you should believe this answer much more).
'and' says to put letters next to each other.
'a sign of the zodiac' becomes 'aries' (Aries is an example).
'diction'+'aries'='DICTIONARIES'
'are in' acts as a link.
Can you help me to learn more?
(Other definitions for dictionaries that I've seen before include "reference books" , "works of reference" .)