For historical facts relevant to UPSC, aspirants can refer to standard sources like NCRT textbooks, Ramesh Singh's Indian History, and NCERT compilations. Government publications and well-known historical texts authored by scholars such as Bipin Chandra provide credible insights. Online resources, including websites dedicated to UPSC preparation, also curate historical content tailored to the syllabus. Regular consultation of diverse sources ensures a comprehensive understanding of historical events and contexts.