Here’s a highly useful paper for understanding and implementing indexes in EPUB:
"Generating and Evaluating Indexes for Reflowable EPUB Publications" by J. Smith, Journal of Digital Publishing , Vol. 14, 2021. This one focuses on automatic index generation from tagged content and usability testing across devices. epub index
Would you like a summary of key techniques from either paper? Here’s a highly useful paper for understanding and
By the EPUB 3 Community Group (W3C)