In my humble opinion if you have an extreemly large library, like I do, the reader is not really relevant what is relevant is how you go about storing the ebooks that you have purchased no matter what format and for what ereader you have. My suggestion is to run out to your local electronics store and buy a 1 1/2 Terabyte hard drive, which is what I have done for my extreemly large ereading library. All of the ebooks are placed on an electronic shelf on the hard drive. I do not think that the ereader makes any difference you will still run into the stroage problem no matter what ereader you have.
I would recommend you describe what your ideal ereader looks like - is it touch screen? Does it have wifi? Does it cost under $100? Does it read every format under the sun? What are your requirements for an ereader?