These are not necessarily the best resources, except for being free. I just kind of picked them at random. If you do a little reading you should get an okay approximation of understanding basically how to investigate things. The simplest approaches that work are always best -- so online searches, simple interviews, and going somewhere and observing something are usually the things to think of first. If you locate any great resources please post them.


FROM THE ORACLE PROJECT:

Criminal Investigation
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Digital Libraries
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Elicitation
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Government Resources (for accessing records)
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Interviewing
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Liaison
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People searches
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Surveillance
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FROM OTHER SOURCES:

How to conduct an investigation
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HOW TO CONDUCT AN EFFECTIVE
WORKPLACE INVESTIGATION
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How To Conduct An Investigation (syllabus)
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Techniques of a Private Investigator
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Private Investigator Surveillance Techniques: Top 10 Tips
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Discovering Private Investigation Techniques
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Skip Tracing Techniques
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Human intelligence (espionage)
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Clandestine HUMINT operational techniques
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BUSINESS INTELLIGENCE: CONCEPTS, COMPONENTS,
TECHNIQUES AND BENEFITS
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Information
Gathering
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Cognitive Interview
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Elicitation Techniques and the Recruitment Process (course description)
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Social Engineering Fundamentals, Part I: Hacker Tactics (talks about elicitation and dumpster diving a little)
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How to Conduct a Surveillance ~ Be Your Own Detective
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Investigative Journalism
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I didn't read the entire article on HUMINT, but I didn't see where they said what it was. They also used the term SIGINT. One stands for Human Intelligence, the other stands for Signal Intelligence. Guess which is which

Just thought I'd clarify that. There're many other intelligence community (IC) terms that end in INT, and some are more difficult to figure out.

HUMINT (Human Intelligence)
IMINT (Imagery Intelligence)
SIGINT (Signals Intelligence)
COMMINT (Communications Intelligence)
TECHINT (Technical Intelligence)
MASINT (Meaconing And Signaling Intelligence)
OSINT (Open Source Intelligence)
TRASHINT (Dumpster Diving)
RUMINT (Rumor Intelligence)

etc

It's made even more complicated by many of them overlapping, for instance COMMINT is SIGINT, but so are some other things.

HUMINT is basically what people think of as detective work or "spying." To further complicate things law enforcement and intelligence services use entirely different terminology. For instance cops say "informant" and intel people say "source." In interrogation they use "themes" in law enforcement and "approaches" in intel. Cops call interviews "interviews," but intel people call them "debriefings."

etc etc.

The practical thing to focus on, especially for most people here who might want to do a little investigating, but not make it their life's work is the core techniques involved. Everybody in the world trying to acquire information, whether for a government, a company, or because they want to find out grandma's secret cookie recipe -- they all end up using the same set of techniques, because they're universal.