It sounds to me the copy of 10.5 install disk is faulty.
Is it a retail copy of 10.5 or have you download/copied a version of this software?
I've known this to happen when the software is a copied DVD disk.
I would also note that if your iBook is suggesting the volume of the install disk can't be verified then,it's more than likely to be a faulty disk or optical drive. I assume you can still boot into the original OS X? If you can it's worth you putting a music or film in the optical drive to test if the drive is at fault.
My problem is similar to yours...
I have an iBook G4 with OS X 10.4.11 on it and I bought a download for OS X 10.5 and I downloaded as a an ISO file which I had to burn to a DVD disc I put the disc in my external DVD drive the instal window opens up and asked to restart to begin the install and when it restarts comes to the current screen where I started.
How can I get 10.5 to install?
The full retail Apple Mac OS X Leopard 10.5 DVD from Apple is the method and license.
Since the Installer is a bootable volume on the DVD, it has to run the computer, or be of significant ability to be seen by the computer as a valid and complete volume capable of the installer, when it is not, it will not work.
And a download OSX version of an older system or even a newer one, may contain errors or parts not originally intended to be in the system, and there was not an official full retail download of Leopard 10.5; only an update Combo 10.5.8 download, requiring a pre-existing correctly installed lower version of Leopard 10.5.
There is almost no way to tell if the download was anything valid, or where the source you got it from, got their illegal copy from. Do not use it.
Good luck & happy computing!