It may be tempting to usability test an extreme case for a complex book search form. Testing extremes is a common and valuable strategy in areas such as engineering, but for a web site, common areas are the most important.
Example: Many sites have both a basic and an advanced search mechanism. It's tempting to task a user with performing an extremely complex query to see if they can do it. However, it's far more important to test to see if they can perform more basic queries first. Most users don’t bother with advanced search anyway.