iBook sharing

ibooks (and other content purchased/downlaoded from Apple) are tied to your account, and they can't currently be lent or copied/transferred to a different account - so unless your friend has access to your iTunes account then you can't lend it.

I'd like to share a book with my wife. If she accesses it through my iTunes account will this put it on her iPad and allow her to read it and keep bookmarks at the same time I'm reading it?

Do you sync to the same computer ? If you do then she should be able to sync it to her iPad. If you use separate computers adn accounts then you are probably better off copying it from your computer to her's - File > Transfer Purchases (File > Devices > Transfer Purchases on iTunes 11) will copy it from your iPad to your computer and you can then right-click the book in the Books part of your iTunes library and click 'show in explorer' so that you can then copy it onto, for example, a flash drive so that you can copy it over to her computer and her iTunes (File > Add To Library). If you re-download it on her computer or directly on her iPad then you risk tying her computer or iPad to your account for 90 days : http://support.apple.com/kb/HT4627

In terms of bookmarks, they are linked to the account on the iPad, so if she uses her account she shouldn't have any problems.

Perfect... that worked!