It depends on what eBook you're trying to download. If you need help, please ask.
EBSCO eBooks can be downloaded and read using an Apple device (iPhone, iPod Touch, or iPad) using the Bluefire reading app available for free from iTunes.
Install Bluefire Reading App. Then, using Safari, open the Library website and checkout your eBook. When prompted, tap to open the eBook with the Bluefire app.