The various types of letter that HR issues are:
Advertisement for job openings,
Interview letter,
Offer of appoinment,
Appointment order,
Probation confirmation letter,
Promotion Order
Transfer Order
Personnel Circulars on Policy changes, Compensation, Welfare amenities
Appraisal letter
Advice Memo,
Disciplinary Action letters like Charge Sheet, Enquiry letter, punishment letter,
Retirement Notice,
Relieving Order etc etc.
How to prepare them depends to a large extent on the company policy regarding the specific HR aspect. Certain formats are available in net, standard books etc.
Any specific format for any aspect could be looked into and drafted.