# Galois theory reference request

I ran into the following description of Galois theory in Gelfand and Manin's Methods of Homological Algebra. But this looks like nothing like the Galois theory I know that deals with subgroups and field extensions. I wonder if there is any great self contained books that treats this topic thoroughly enough so that I can understand the following paragraphs? Thank you.

• Since only non-empty open subsets and non-empty intersections are considered, the sheaf condition doesn't give $F(U \sqcup V)=F(U) \times F(V)$. – Martin Brandenburg Dec 11 '14 at 8:41