For a long time I've wanted to backup my ebooks and audio books using a free cloud service, but I found that none of them offered enough free space to meet my needs. I can't afford to pay for a premium service so I just settled for backing them up on an external hard drive. I just found out about Copy.com though and I finally found a site that seems to meet my needs so I can enjoy an extra backup on top of my external hard drive.
Before I found Copy.com, Google Drive seemed to offer the most free storage with 15 GB of free space. Copy.com has matched that and if you sign up through someone's referral link they give you a bonus of 5 GB for a total of 20 GB.
Finally a site that offers enough free space to hold all of my ebooks and audio books(though just barely).
In addition to earning a bonus of 5 GB of space when you sign up from someone's referral link, you also get an extra 5 GB of space for every person you successfully refer. This blows Dropbox's referral program out of the water and there seems to be no limit to how many people you can refer.
Another great feature of Copy.com is that if you share your files with other users, then the data gets split equally between all accounts the files are shared across. So in addition to getting bonus space from referrals you can get more space by signing up for multiple accounts and share/sync the files across all accounts. You won't get a referral bonus for your extra accounts though unless you install the app on separate devices but you can install on both a PC and a tablet/phone. I've registered for 4 accounts myself and it really does work. With the extra accounts I then had room to backup my music/pictures/documents as well.
So keep your ebooks safe and Copy them. Remember to sign up with a referral link in order to get an extra 5 GB of space.
I won't be offended if you choose to sign up without using my referral link as I already have plenty of storage space, but you should use someone's else's referral link if not mine for an easy extra 5 GB.