A letter of hire is a good way to let a new employee know that the NPC is the hiring agent and where the employee will need to go if they have questions regarding their employment. Information in the letter of hire should include start date, position title, starting wage, employment status, etc.  To preserve "at will" status, avoid words like "permanent employee," "probationary period," or any other statements that could make it difficult to discharge an employee "at will."