Basically, it's just a question of locating the right directories where ADE and/or Sony EBL will have put your files, and then copying it to the device, after it has been authorised.
IMPORTANT: it just works on PCs and Macs. I'm not sure if both programs are supported by emulators (like Wine).
Step by step procedure goes more or less this way:
1. Depending on what source of ePub/PDF DRM-ed files you are aiming at (your store) download Sony EBL (EBook Library) software (without which you can't have access to the Sony store) and/or Adobe Digital Edition (for any other store and for lending libraries) from their own sites.
2. After installation, register both for the Sony store and for an Adobe Digital Edition ID- in this order AND from the Sony software, or you'll get in the same mess as I if you used perchance two different e-mail addresses. Sony should, I think, automatically create you an ADE ID.
2.1 Not in the US? Either you purchase with the help of some forum member based in the US a gift card for the Sony Ebook Store (it's different from the regular Store; check the Flea Market or the Sony sections of the forum for any detail) or you tinker with your CC's details (again, information can be found in the Sony forum- technically it's not completely ok, but it does work).
3. Buy your books
4. Localise the folder My Books (usually in the Documents folder): the ePubs are there.
5. Open ADE and register with the same ID as in your Sony account, if prompted. Connect the PB and authorise it in ADE. (You can't do this in Sony EBL because it simply won't recognise your PB as a valid unit/reader)
6. Close ADE and drag-and-drop in Finder/Explorer the books from My Books. Disconnect the PB.
You could also use drag-and-drop from ADE if you're buying from other sources (and it works with PDFs encrypted with ADE), or having a book on lease from your local library. Mind you, I can manage just fine buying from, say, Waterstones', and having the "protected link" file sent to Sony EBL, which is slightly less annoying (i.e. slow) than ADE; then I take the files from My Books and here I work.
One last word: if you don't want to enter in the world of decryption/dedrm-ing/piratage or how you'd like to call it
, be mindful of the number of your ADE authorizations- max 5 devices authorised/account. You can't manage them if not by un-authorising the device by hand and directly (i.e. not on a control panel, like with Sony or Kindle authorization), and if there's some mistake you'll have to have a lot of patience and e-mail Adobe Customer Service so that they can put to zero the total of your authorised devices, and it can take some time- it took me a week, which wasn't fun at all (so I'm happy there are free sources of reading material and publishers that will use other methods, like "tattooing" your pdfs- yep, can be a bit irritating at first, but with FBReader, on your PB, you can leave the tattoo out of the part of the screen you're looking at).
If the "tutorial" needs some correction, please comment and I'll change the text where appropriate
p.s. even on orthographic side; I'm under the beginning of a slight migraine and my spelling isn't the best in these moments. Er, back to not backlit pages- in other words, to my PB