Search usability

Been working with a team that is assessing usability of database search pages using a Web interface. Thought it migt be helpful to capture the current literature we are referring to. Have any other suggestions that you have used and find valuable? Nielsen/Norman Group. "Search: Design Guidelines for E-Commerce User Experience". -- $45, I would say it is worth it to buy this. Nielsen, Jakob. "Search and You May Find". Spool, Jared. "Web site usability: A Designer's Guide". -- 1997 study that tested 7 sites for usability issues -- one of the sections is search.