Starting immediately, the free price tier will offer 15GB for the low price of nothing, and if that's not enough you can get 100GB for only $1.99 per month (down from $4.99). The upper tier of one terabyte is now only $9.99 a month (down from $49.99), and if you need more you can get additional storage, in 1TB segments, for $9.99 a month.
The new prices are considerably cheaper than some of Google's competition. For example, Dropbox is charging $9.99 a month for 100GB, and they only offer 2GB free.
There was a time where I had my ebook library backed up to Dropbox, but there came a point where it grew to big to fit into the free 2GB. And since I didn't find the idea of paying for storage all that appealing (not when I am going to carry my ebook library around in my laptop anyway) I largely stopped using Dropbox.
Maybe it's time I looked into Google Drive. I'm currently only using about 3GB of the free 15GB allotment, and that is mostly taken up by my Gmail account. That leaves a lot of room for an ebook library.
Google has Google Drive apps for Android, iOS, OSX, and Windows, so syncing your ebook library to the cloud is as easy as simply setting it up the first time.
You can find more information on the Google Drive website.