OSINT Techniques: Resources for Uncovering Online Information (11th Edition)
by Michael Bazzell, Jason Edison · IntelTechniques (self-published)
This is the definitive single-volume manual for technique. A 590-page practitioner manual on searching for people, usernames, email addresses, phone numbers, domains, images and social networks. It also shows how to build a dedicated investigation virtual machine and your own custom search tools, and how to document and report findings.
More resources on OSINT (Open-Source Intelligence)
Berkeley Protocol on Digital Open Source Investigations
International standard (2020) for professional open-source investigation. It covers legal and ethical principles, investigator security, online inquiry, collection and preservation of digital material, verification and analysis, and reporting findings so they hold up as evidence. This is the authoritative text for the ethics and evidentiary-methodology side of the topic, which technique-focused resources skip.
Bellingcat's Online Open Source Investigation Toolkit
Reference of investigation tools grouped into maps and satellites, geolocation, image and video, social media, people, websites, companies and finance, transport and archiving. Many entries include a write-up of how to use the tool and its limitations. It is a tool directory, but it is the canonical one: actively maintained, reviewed, and annotated with usage notes and caveats.
OSINT At Home: Tutorials on Digital Research, Verification and Open Source Investigations
This is the best free, hands-on starting point for the core skills (search, image and metadata work, geolocation). Short screen-recorded tutorials, each on one investigative technique: reverse image search, finding EXIF metadata, advanced search operators combined with translation, geolocating a photo or video, and dating imagery with satellite history.