Employs ghostwriter perhaps responsible for a number of tomes (4,9)

'employs ghostwriter perhaps responsible for a' is the definition.
I know nothing about this answer so I can't judge whether this works.

'number of tomes' is the wordplay.
I cannot really understand how this works, but
'number' could be 'l' (L is an example) and 'l' is found in the answer.
'of' could be 'o' and 'o' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?