[wallcraft]

Sharing is entirely based on the DRM scheme, and is essentially the same no matter where the ebooks are bought.

For Secure Adobe (PDF and ePub), the DRM is keyed to a single Adobe ID. You can register up to 6 devices per Adobe ID. If you have Sony Readers, then this applies to them too (all Sony's registered to the same Adobe ID can read the ebooks).

For MOBI the DRM is keyed to up to 4 devices, which are listed on your account. It is perfectly ok to include other peoples devices.

For LIT, the DRM is keyed to a MS account (I think). Probably 95% or more of LIT downloads have their DRM stripped immediately and are then format shifted to some other format. So its audience is format shifters rather than Secure ebook sharers.

For eReader there is no device limit, but the key is your credit card number.

Note that an advantage of Adobe and MOBI in particular is that you and your family members can each have your own BoB account and just use the same DRM identifiers (list each other's devices, or register the same ID).