How are you trying to read them?
There is an app called Bluefire (and other similar ones) that can read any Adobe DRM books, including library books. You need to download the library books onto ADE on a computer, then copy across the resulting .epub file (sometimes .pdf) to the tablet. You will find a place in Bluefire to register your AdobeID.
Alternatively, there is an Overdrive app. That will let you borrow, download and read library books with no need for a PC/Mac or for ADE.