What is the "8900" that the FAA uses for guidance?
The Flight Standards Information Management System, created by FAA Order 8900.1, is the source of all information relating to aviation safety inspector job tasks.  It's about 8,000 pages worth of (non-regulatory) guidance and policy helping FAA Inspectors to do their job properly.

It's an electronic-only document, which can be found at fsims.faa.gov, or by visiting the FAA homepage and entering "8900" in the search box at the top right.

There's all kinds of good stuff in there, such as:  


How do I give a practical test for a CFI applicant with a sport pilot rating?
How do I perform field approval of a major repair or alteration?
How do I do a ramp inspection?


And lots more!

FSIMS is open to the general public - if you want to know what an inspector does day in and day out (or what to expect when you meet one), FSIMS is the one-stop-shopping location for the answers to all your questions.