Group books about reversing and so on (5)
I believe the answer is:
octet
'group' is the definition.
(group of eight musicians)
'books about reversing and so on' is the wordplay.
'books' becomes 'ot' (Old Testament).
'about' indicates putting letters inside.
'reversing' says the letters should be written backwards.
'and so on' becomes 'etc' (synonyms).
'etc' backwards is 'cte'.
'ot' going around 'cte' is 'OCTET'.
(Other definitions for octet that I've seen before include "A group of eight instruments or voices" , "Musical work for eight" , "Set of eight" , "A number of musicians" , "What takes members up to eighth" .)