Articles about assessing discovery services
I began with this one, which includes the others in its bibliography.
Library Discovery Products: Discovering User Expectations through Failure Analysis http://ejournals.bc.edu/ojs/index.php/ital/article/view/9190/pdf
1. Bess Sadler, “Stanford's SearchWorks: Unified Discovery for Collections?” in More Library Mashups: Exploring New Ways to Deliver Library Data, edited by Nicole C. Engard, 247–260 (London: Facet, 2015).
2. Jody Condit Fagan et al., “Usability Test Results for a Discovery Tool in an Academic Library,” Information Technology & Libraries 31, no. 1 (2012): 83–112.
3. Megan Johnson, “Usability Test Results for Encore in an Academic Library,” Information Technology & Libraries 32, no. 3 (2013): 59–85.
4. Andrew D. Asher, Lynda M. Duke, and Suzanne Wilson, “Paths of Discovery: Comparing the Search Effectiveness of EBSCO Discovery Service, Summon, Google Scholar, and Conventional Library Resources,” College & Research Libraries 74, no. 5 (2013): 464–88.