Reciprocal Rank Fusion (RRF): A Multi-System Combiner, Jeffrey A. Allan, Bruce Croft, 2009Proceedings of the Eighteenth Text REtrieval Conference (TREC 2009) (ACM)DOI: 10.1145/1879792.1879809 - This is the original paper introducing Reciprocal Rank Fusion (RRF), a method for combining ranked lists from multiple search systems.
Okapi at TREC-3: Automatic Query Construction and Other Experiments, Stephen E. Robertson, Stephen Walker, Michael Hancock, Ann Beaulieu, Mike Gatford, 1994Proceedings of the Third Text REtrieval Conference (TREC 3) (National Institute of Standards and Technology (NIST)) - This paper formally introduced the BM25 ranking function, a widely used algorithm for keyword-based information retrieval.
Introduction to Information Retrieval, Christopher D. Manning, Prabhakar Raghavan, Hinrich Schütze, 2008 (Cambridge University Press) - A foundational textbook covering information retrieval principles, including keyword models like BM25 and strategies for combining search results.