Sailor with old books that may have mystical signs (5)
I believe the answer is:
tarot
'that may have mystical signs' is the definition.
(a Tarot deck is used to tell the future)
'sailor with old books' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor).
'with' means one lot of letters go next to another.
'old books' becomes 'ot' (Old Testament).
'tar'+'ot'='TAROT'
(Other definitions for tarot that I've seen before include "fortune-teller?" , "Cards often used in fortune-telling" , "Mystic cards" , "Fortune-teller's card" , "These cards are used by fortune tellers" .)