TITLE: References to SGA 8 and descent theory
QUESTION [9 upvotes]: In Geometric Invariant Theory, by Mumford, Fogarty, and Kirwan, if there is a mention of descent theory, it almost always comes along with a reference to SGA 8, Theorem 5.2 (see the end of the proof of prop 6.9 on page 119, for example).  Now I know SGA 8 was never made, but I was wondering:

Does anyone have a good guess as to what this theorem should say?
Does anyone have a good reference for a quick and "hands off" introduction to descent theory?  I am really just looking to understand the "gist" of it.

REPLY [5 votes]: For question 1, see the comment above.
Collecting the answers to question 2:

Grothendieck's original FGA, starting with TDTE I
Vistoli's chapter in FGA explained, for the connection with stacks
What is descent theory? for a very short overview
Bosch-Lütkebohmert-Raynaud, Néron Models (recommended by BCnrd in the above-mentioned thread)
Waterhouse, Introduction to Affine Group Schemes, containing a 20-page introduction primarily concerned with the affine case

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