While many E-Book formats exist, EPUB is emerging as the industry standard, readable on most major tablets and eReaders except the Kindle (which uses Amazon's own proprietary format). The newest version of EPUB allows for compatibility across a wide variety of devices and provides publishers with formatting control, interactivity, and accessibility features.
See the EPUB site for more: http://idpf.org/epub
Each device supports different formats. For more information on what formats are provided by the library, see Using E-Books.
Nook, Sony, Kobo and more: These E-Readers will work with the widest variety of downloadable library E-Books. PDFs may be easily transferred to them, and they are compatible with Adobe Digital Editions. If you own one of these E-Readers, you shouldn't have many problems getting content onto your device.
Amazon Kindle: Unfortunately, the Kindle is not compatible with Adobe Digital Editions. However, you can still transfer PDFs to the E-Reader, so there are many library E-Books to read on this device.
iPad, iPhone and iPod Touch: Compared to most E-Readers, these devices provide a much better reading experience online. They are also excellent for reading PDFs. You can also download an app like Bluefire (http://itunes.apple.com/us/app/bluefire-reader/id394275498?mt=8) that is compatible with Adobe Digital Editions.
Android phones and tablets: Compared to most E-Readers, these devices provide a much better reading experience online. They are also excellent for reading PDFs. You can also download an app like Aldiko (https://market.android.com/details?id=com.aldiko.android&feature=search_result) that is compatible with Adobe Digital Editions.