I have a Sony Ereader since Christmas I would highly recommend it.
There is an excellent cross-platform open source free library management application called Calibre which I use to add and delete books to my Ereader. Calibre supports conversion from (HTML, LIT, RTF, TXT, PDF(simple text+few images)) to the SONY ebook reader format LRF and it is regularly updated. It works with the Sony firmware.
You can download it here:
It doesn't work with 64 bit Windows.