Electronic – find documentation on Eagle XML file structure

The best answer is to make your own, it's easy. Someone else's library parts aren't going to adhere to your conventions for the silkscreen, hole sizes, BOM handling, etc, etc. I never even look in the Eagle libraries as they are basically useless.

In your case, you haven't even said what kind of button, only shown the schematic symbol. There are many many many different "buttons" from various manufacturers. Most of these will require a different footprint. You have to specify a particular part before you can even tell if someone else's Eagle button is applicable to your case.

In the off chance you can live with my conventions, download my Eagle Tools release from http://www.embedinc.com/pic/dload.htm and look around in the "Switch" library. There are some buttons in there, but again, there is no way to know what you actually want.

Starting with v6, Eagle supports true copy&paste. This means that you can copy any part from one design to the windows clipboard and just paste it into another!
You can even copy/paste groups of parts, and between two different eagle instances (this wasn't possible before either).

This is a MAJOR improvement from earlier eagle versions (where the cut/copy/paste was totally counter-intuitive/horrible) and reason enough to upgrade.

Note that you have to use group select + copy even for a single party to make it work. The just paste it in any other schematic.

This way you only "import" the parts into the schematic and don't have to worry about from which library it comes or where it should go at all (assuming this is not a requirement).