hi casi and welcome !
as others have said, the cybook will work with linux as it is seen as a usb mass storage device. actually, the sony 700 will also, for the same reason, and i believe also the 505. there may be even more options available to you ! the sony devices will read the epub format natively, and you can easily convert mobipocket so those books are available to you as well.
whatever device you choose, since you are on linux, i recommend you take a look at
calibre for your library management and format conversion : it is available for linux, and it is really handy for managing your ebooks and also for rss feeds (among other things !). i use it exclusively, although i am using a windows pc.
i'm sure you will be really happy with whatever device you choose !