Whenever you buy an "ePub" that is copyright protected, the publisher incorporates some sort of DRM (Digital Rights Management) into the file. Among other things, it determines what devices can and can not be used to read the file. In other words, if you buy a book from Amazon, it can only be read with a Kindle device or with the Kindle software or app.
The OverDrive system uses Adobe Digital Editions. The DRM system Adobe uses is called ADEPT (Adobe Digital Experience Protection Technology) and it uses Flash-based technology. If you are not aware of this, Apple mobile devices (iPhone, Ipod and iPad) can not use Flash. (This is a topic severely beaten to death on this forum and elsewhere!) Therefore, it is unlikely that you will ever be able to check out a library book with the OverDrive system as long as they use Adobe Digital Editions.
The good news is that it is technologically possible to "unwrap" the ADEPT from Adobe Digital Editions thus making them a "generic" ePub that can be read on just about any reading app. Is this legal? I am not a lawyer, but I am sure the lawyers at Adobe have a very definite opinion on this!
By doing some diligent searching on the Internet, you should be able to find out how to do this.
