Potter billionaire held back music book (8)

I believe the answer is:

libretto

I'm a little stuck... Click here to teach me more about this clue!

'music book' is the definition.
'libretto' can be an answer for 'book' (libretto is a kind of book). I am unsure of the 'music' bit.

'potter billionaire held back' is the wordplay.
'held' indicates the answer is hidden within the clue.
'back' says the letters should be written in reverse.
'otterbil' is hidden within 'potter billionaire'.
'otterbil' backwards is 'LIBRETTO'.

Can you help me to learn more?

(Other definitions for libretto that I've seen before include "text for musicians" , "Words set to music, especially opera" , "Book of words of an opera" , "Text written for an opera" , "Operatic text" .)

I've seen this clue in the Sydney Morning Herald.

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