Open Source Intelligence Techniques — Michael Bazzell
inteltechniques.com
Official site with book updates, podcast & tools (current edition available on Amazon)
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NIST SP 800-150 — Guide to Cyber Threat Information Sharing
csrc.nist.govOfficial US gov PDF, free
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Related in Extra Resources
Bellingcat Resources
bellingcat.com
Free guides, case studies & methodology articles
WebFree
Bellingcat Online Investigation Toolkit
bellingcat.com
Live, searchable toolkit maintained by Bellingcat
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SANS Reading Room — OSINT
sans.org
Peer-reviewed whitepapers (free with registration)
WebFree
SANS Reading Room — Forensics
sans.org
DFIR whitepapers, free
WebFree
SANS SEC497 OSINT Course
sans.org
Course outline, free sample content
WebFree