Elizabeth and I have verified that you can share iBooks across itunes accounts. Here is how it works:
You need to authorize the two computers you sync your iDevices with to each other’s account. In other words, my machine is authorized for her iTunes account, her machine is authorized for mine. You do this via the Store menu in iTunes.
Next, you need to turn on home sharing (Advanced menu) on one of the machines. This will be the “master” account.
On the other machine, turn on home sharing, but enter the itunes account of the master account.
Now, in the left hand sidebar, you’ll see the other itunes account under Sharing, and the other account will see you.
You can open up the account with the disclosure triangle and drag content into your itunes. Drag the content to the upper part of the left hand bar – You’ll see it hilite.
Sync your iDevice with iTunes and enjoy the apps, music and books of the other account!