Some clients don't need a manual, but writing one will save you a lot of time and effort down the track when you get the inevitable phone call asking something simple like "How do I get a page out of the trash?"
There are decent online resources for end users and recently eZ Systems have released a book to help with this—eZ Publish Content Management Basics. However, even if you provide a manual and a copy of the book, you should expect that you will still receive phone calls from people who aren't technically strong, and struggle even with the aid of a manual.
Even for people who have been through training, simple tasks that they don't do often will be forgotten. Some clients are smart enough to work it out for themselves but most will call you for help, that's why it's a good idea to provide them with a user manual; it's also good customer service.
There are two parts to the user manual, standard features and custom features. Standard features include everything that comes with...