Yes, if you go to the Google Music website for your music library, you can select an album to download to your computer. It will only let you do this twice for each album, so keep any downloads somewhere safe.
If you use the Google Music Manager program on your computer, you can download your entire library an unlimited number of times, but the catch is that you have to either download it all, or just your free and purchased music.
EDIT: Hmm, just realized that I essentially repeated what I said a few posts back!
Amazon seems to have more sales, but I think Google music has higher bitrate tracks (320 kbps).
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