If i lose my iTunes library, can i "rescue" my iTunes purchase?

Apps and ibooks can be re-downloaded in all countries, but music, tv show and film re-downloads are only possible in some countries : what can be re-download where and re-downloading. But even if you are in a country where films can be re-downloaded, not all film studios have agreed to it, so not all of your films may appear for re-downloading - and there is no guarantee that a film (or app, album etc) will remain available in the store, content providers occasionally remove their items.

You are better off taking a backup of all of your library, rather than relying on being able to re-download them in the future.

It has always been very basic to always maintain a backup copy of your computer.

It has onlly be very recently that you have been able to redownload anything at all.

The movie industry would have to allow this.

If you buy a dvd from a store and lose it, you do not get a free replacement. Why would a donwload be any different?

It is your responsibility to backup your content. This has always been the case.

You can re-download an app as long as it remains in the store - you should also keep your own copies of them as app developers (and Apple) occasionally remove them from the store. With apps there is also the potential issue of their requirements e.g. if your device only supports up to iOS 5 and the developer then releases a version that requires iOS 6, then you won't be able to install the store version.