Book found by a sailor travelling north (5)
I believe the answer is:
atlas
'book' is the definition.
(atlas is a kind of book)
'by a sailor travelling north' is the wordplay.
'by' is a charade indicator (letters next to each other).
'sailor' becomes 'salt' (salt is a kind of sailor).
'travelling north' says the letters should be written in reverse (in a down clue, letters go up).
'salt' in reverse letter order is 'tlas'.
'a'+'tlas' is 'ATLAS'.
'found' acts as a link.
(Other definitions for atlas that I've seen before include "The first cervical vertebra" , "North African mountains - mythical Titan" , "Large-format book" , "I carry a huge weight" , "It appears at the top of the spine" .)