Sounds basic but many solo and small firms do not have one. You need one to act as a reference for both management and employees alike. It can be your shield against discrimination, harassment and other claims. It can save time by having answers to common questions on routine employment matters and issues and employee concerns in such areas as vacation and sick-leave policies, unpaid leave, holidays, work hours and other such details. It can also contain forms (evaluation forms), descriptions (jobs, tasks and expectations) and procedures (fire, disaster, privacy etc).
Most office manuals today also address emerging issues such as: internet usage, social media use and privacy expectations.
A good (albeit American) article on how to write an office manual is available from The Lawyers Mutual Insurance Company of North Carolina.
The Law Society of BC has a number of Model Policies that can be incorporated into an office manual.