Epistemic interaction refers to the way that intelligent user interfaces can be designed so that user interactions reveal more information about the users preferences or goals and thus improve future interactions. For example, if the user hovers over a glossary entry in the text it shows they have some interest in the topic, whereas if the entries are permanently shown in a sidebar it is impossible to know which the user has read. The former may be prefered as it provides better knowledge, even if marginally less usable.
Used on Chap. 19: pages 483, 485