Walk on books? That's harsh! (8)
I believe the answer is:
strident
'harsh' is the definition.
(I've seen this before)
'walk on books?' is the wordplay.
'walk' becomes 'stride' (stride can mean to walk around).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'books?' becomes 'NT' (abbreviation for New Testament).
'stride'+'nt'='STRIDENT'
'that's' is the link.
(Other definitions for strident that I've seen before include "Shrill" , "Harsh, grating" , "Offensively loud and harsh" , "harsh sound" , "Unpleasantly loud and harsh" .)