What do you mean by 'on the books'?
If you wish to pay the sub contractor under the CIS rules then the previous answer is fine but if you mean you wish to take them on as an employee on PAYE basis then you have to consider the following as well:-
You will be liable for Employers NI.
You will be liable for Holiday Pay.
You may be liable for Sick Pay depending on contract terms.
You may be liable for Down Time Pay depending on contract terms.
It is more difficult to sack an employee. With a sub contractor you just do not give them any more work.
You may be liable to Redundancy Payments.
These are the main points to consider.