Nexus 7 tablet and ibook library

Unfortunately, you can't. The iBook Store and items purchased from it are for Apple mobile devices running the iOS operating system. The Nexus 7 is an Android device and cannot run or use things designed for iOS devices.

If you wanted a tablet for reading your iBooks Library you needed to purchase an iPad.

Of course making use of multi-platform applications can be a benefit, but then companies only ever design for the lowest common denominator, so you never get apps that take full advantage of any particular platforms unique or enhanced features, and technology advances never get pushed forward if everything is the same everywhere.

Apple seems to prefer publishing the best apps it possibly can on its own platform to make it the best, rather than lowering their standards by making things compatible with competing platforms.

You could use iCloud with a shared iCloud account (although iCloud is really intended for personal use), but if any of the individual users also has their own personal iCloud setup on the device/computer, it could get a little tricky to manage. There is no problem having multiple iCloud accounts setup, but there are limitations:

http://support.apple.com/kb/TS4020

If you only want to sync Contacts (and keep them separate to users personal contacts), perhaps Google Sync or Fruux account would be better:

http://support.google.com/mail/bin/answer.py?hl=en&answer=2753077

https://fruux.com/

They'll work independently and co-exist happily with personal iCloud accounts.