I'd delayed updating to iTunes 9.2, but the upgrade seems to have gone well.
... the iBookstore is only available through iBooks on iPad, iPhone, and iPod touch at this time.
Books downloaded from the iBookstore can be placed on up to five computers you own that you’ve authorized with your iTunes Store account. You can sync your books to all iPads, iPhones, and iPod touches you own...
Apple says "five computers", not "five user accounts". A single computer can have a very large number of separate accounts, and each account can have a distinct iTunes account.
From past experiments I believe it is correct that Apple authorizes at the computer level, not the user level. The FairPlay DRM however is iTunes Store account specific. I don't think there's any documented limit to how many user accounts can share the same iTunes account, so in principle a single machine could have 100 iTunes accounts that could all share the same iTunes store account and thus the same FairPlay material. In turn each account can sync to an unlimited number of iDevices (I thought there used to be a limit!) -- though technically one person should "own" them. (How is that established?)
This DRM implementation means that DRMd material can be shared by a group of people so long as one person is making all the purchases ("ownership"). It's not a bad proxy for a "family of residence". So books can be shared within a family, as can music, video and apps.
It's never noted anywhere but on my blog, but this makes iPhone apps much cheaper for families than apps on other platforms (example: Wii).
The strategic impact of Apple FairPlay is underrated. It seems to suit everyone to keep it that way.