Let me get this straight - you're using the iMac as a base station so that your TiBook can bounce off the iMac and get to the internet?
From your description, I believe you want the ethernet connection to be the first one listed in the iMac's Network preferences. This is the connection that the iBook uses first when initiating connections. In addition to that, since the AirPort card is only handling local traffic, make sure you do NOT have a default gateway/router defined on the iMac's AirPort configuration. Only the ethernet connection should have a default router set.
If that doesn't fix it, you need to provide more details, such as whether the iMac is running as a bridge or a NAT device - are the same subnets used on each side of the iMac? Are you using DHCP to serve addresses, or static addresses on the iMac and TiBook?
From your description, I believe you want the ethernet connection to be the first one listed in the iMac's Network preferences. This is the connection that the iBook uses first when initiating connections. In addition to that, since the AirPort card is only handling local traffic, make sure you do NOT have a default gateway/router defined on the iMac's AirPort configuration. Only the ethernet connection should have a default router set.
If that doesn't fix it, you need to provide more details, such as whether the iMac is running as a bridge or a NAT device - are the same subnets used on each side of the iMac? Are you using DHCP to serve addresses, or static addresses on the iMac and TiBook?