Of course, the software should be easy to use, though this is a hard quality to evaluate. For example, WordPerfect is a non-standard, sometimes cryptic word processor, but it is also very powerful and very popular. It could be recommended even though its ease-of-use rating would be low.
Our assumption is that the potential hyperbook author is not a computer expert, but is willing to learn a program equal in complexity to a simple word processor. The presence of sufficient documentation, either on paper or in the form of context-sensitive help, is essential.