Make sure you got a Sony 650 versus the 600. The 600 had a screen that was more difficult for reading. The 650 is gorgeous and clear. If you did get a 600, I'd take it back.
Also, take a look at the matrix, located at
https://www.mobileread.com/eink/. It shows all of the readers on the market right now. Any of the ones with Adobe Digital Editions will work with your library books. Once you narrow it down to a few readers, you can post specific questions to the forums for those readers. There are readers that no one has ever heard of with great customer support here at MobileRead.
Having said all of that, I love my 650 for the screen and the ease of changing pages. My arthritic hands find the swipe on the touchscreen easier than pushing buttons.