Whether you can download an older compatible version of an app depends upon whether the app's developer allows it.
For iBooks try downloading the current version on your computer's iTunes so that the app is in your purchase history. Then on your iPod go to the Purchased tab in the App Store app and you should be able to download an older compatible version of it. You can also try similar for other free apps that you want. For paid-for apps try contacting the developer first to check if that will work, rather than spending money first and then finding that you can't.