Employees come and go, but business must go on and go on efficiently. Each department needs to have written procedures. This is not so much an employee handbook, but more of a detailed step by step explanation of specific tasks within departments.
In most companies when there is more than one person in a department, each employee will have tasks and responsibilities that are assigned to them. If that employee is out sick, on vacation or leaves, other staff members from the department will need to be able to complete their work so production doesn't lag. There is nothing worse than for an employee to quit and no one else has a clue how to do that employees work.
Keep a list by department what the job responsibilities are, then write out a step by step process for each task that workers within the department are responsible for. Take for example; in the accounting department, the person who processes credit reference requests. What forms are sent out? How are they sent? How do you keep track of the credit reference requests sent; so you know who's answered and who you need to follow up with? Who gets the completed forms to make the final credit decision?
If the company is uniform in how things are completed and workers within the departments are cross-trained; when someone is on vacation, out sick or quits, the company won't suffer.