@quisvir: Thanks! Worked like a breeze! My faith in humanity has been restored. It just took me 5 minutes to buy this e-book. Got a bit confused by the .acsm file format, but ended up with a ePub soon enough.
However, I still can't read it on my Sony Reader because of this problem:
https://www.mobileread.com/forums/showthread.php?t=59083
Aargh!
Update: half an hour later. Went through the whole process of installing ADE again on my little Windows netbook and I successfully authorized my Reader and transferred the ebook. It works, but it sucks big time I cannot transfer DRM'ed ePub ebooks from my two main computers, both Macs. I thought this "PC-only" thing was way behind us in 2009.