By law, the prompt book belongs to the company. In many shows, it is in the contract signed when leasing the rights to the show, that the COMPANY may keep ONE copy of the script, in the form of a prompt copy. Most companies will not let the prompt copy go, and will insist on keeping it.
Before I turn my prompt copy back over to the company, I shred all private documents (including medical forms), and if the company provides a photocopier, I will copy the entire thing. If not, I bind them using legal ribbon and keep my folder for another day (A decent sized four ring binder is extreamly hard to find and certainly not cheap)