Kobo uses a special ebook format called Kpub or something like that. It is not compatible with anything but a Kobo reader.
You must go into your library on the Kobo website and download the books to your computer. The downloads will be in
an .asm format. This is the format that is needed for Adobe Digital Editions. You will need to open them with ADE and
from there you should be able to export them to your Sony. I don't know Sony products so I can't help you with that.
BTW you can use the ADE without it being authorized.