OS 10.5.8 imac
I recently changed all my itunes to MP3s.
I thought I would create a new library of AIFFs.
After creating a new library, my original one disappeared.
When I inserted a CD, they downloaded as MP3s and not AIFFs.
Fortunately I have backup to restore my HD to as it was.
Q: where did my original itunes go why did the inserted CD turn out to be MP3s?
jaybass

How songs are imported into iTunes is set in the "Advanced" section of "Preferences" in iTunes, and it is independent of the format of the songs in your Library.

Questions: how did you create the new AIFF library? And where were you looking to see that the original one had disappeared?

I created a new library by holding down the option key and clicking itunes at the same time.
I then clicked create library. My itunes disappeared and they were no-where to be found.
When I inserted a CD of songs, they appeared as AIFFs.
What I want is a separate library of AIFFs only.

jaybass

You correctly created a new Music Library. Your old Library is still where it should be--don't panic, you just access it by holding down the option key while starting up iTunes. The same dialog that allowed you to create a new library will now allow you to choose between the two libraries you have.

Your new library is currently blank (or nearly so). Set your preferences in iTunes so that when you import music you do it as AIFF. If you want to take your old library and convert it entirely or selectively to AIFF format that is another thing altogether.