Not all site-specific art is installation, but I am hoping to articulate here the idea that the two are so linked that installation art is site-specific, by nature of its medium.
I would argue that there are three main types of site-specific art.
There are those that are site-specific, although they might not be defined as such because they cannot be moved; works by Robert Smithson for example. They are specific to that site.
Then there are those that are site-specific because they respond to the site, they are indelibly linked through subject as well as presentation. (This is the traditional definition of site-specific art)
And then there are those that can be defined as site-specific because they change and shift based on the site in which they are presented (which could be argued to be a response to the space, but it remains distinct from the previous type.) Each incarnation of these works is specific to that site because it relies upon it.
Installation comes into this category, although it can certainly fall into the others as well. Installations, at least in my work, are a collaboration between artwork and space. They are temporarily site-specific.
No bibliography for these notes, they are thoughts occurring alongside or inspired by my research (primarily 'studio research').