Stone Spaces, Locales, and Topoi for the (relative) beginner

I cannot praise MacLane and Moerdijk's book "Sheaves in geometry and logic" highly enough.

Many good books have already been mentioned; I like MacLane+Moerdijk as an introduction, and after that both books by Johnstone (in particular, Part C of the Elephant does a good job of connecting locale theory with topos theory). But I also wanted to mention Vickers' paper "Locales and Toposes as Spaces," which I think does a good job of connecting up the topology with the toposes and the logic in a way that isn't directly evident in many other introductions.

The book by Robert Goldblatt: "Topoi, the Categorial Analysis of Logic" (1984) is quite simple to follow and have the nicely consulted at this address: http://historical.library.cornell.edu/cgi-bin/cul.math/docviewer?did=Gold010&id=3