[FuzzyDunlop]

I think this quote sums up some of the thinking behind this:

"Some would say that a repository is storage — a place where you store your code — somewhat like a box."

So why isn't the icon a box? Why instead spend the rest of the paragraph justifying the book icon? It might be understood well enough by the Github designers, but the metaphor isn't universal enough to be recognisable at a glance.

This is made worse because the icon for 'readme' and 'wiki' are also books. That could be confusing without the text.

The other example is the icons for watching. 'Add' tends to be represented as a plus icon. In this case 'watching' has a plus icon in it, while 'unwatch' has a cross icon. This doesn't appear to make sense.

One might wonder why, when there are present and available dichotomies (open eye, shut eye), there's such a mix and match of metaphors.