Jellyfin 12.0 is a major stable release that drops the "10" version prefix and focuses on performance improvements, database optimizations, and enhanced book/comic support. Key features include faster library browsing through improved playlist and collection storage, alternate versions support for TV episodes, built-in book metadata reading with a redesigned interface, customizable recommendation sources, and the Modern layout becoming default. Users must backup before upgrading, run a full library scan afterward, and ensure they're on version 10.10.7 or newer before updating.