Grab one and you can flop it open in the middle, skip around, and thumb through its pages forwards or backwards. You can consult it in all sorts of odd sequences and quite often you do. With the index, a 500 year-old tool, you can hop around from topic to topic, nearly instantaneously. Publishers have long designed dictionaries, directories, and encyclopedias specifically for non- linear access, and reference books work well. Our fingers quite naturally apply enhanced binary-search algorithms on such volumes as we look an entry up.
Books have little in their structure that is inherently linear.
Proponents of hypertext should pay close attention to this fact. Encyclopedias are interactive documents. Writers craft entries for them assuming no continuity with what lies before and after. They include numerous cross references, links in the jargon of hypermedia. They are most useful works, but rarely do they hold us spell-bound by the suspense of the tale or the force of the argument.