Many Aperture users are finding multiple libraries to be the key to their organizational woes. As Derrick Story described here, Aperture 1.5 provides a simple way to switch between multiple libraries.
However you choose to make the switch from one library to another, it’s worth noting that Aperture stores all of your keywords in the Application Support folder inside your user Library folder. Every time you launch a new library, Aperture scans the keyword database in your Application Support folder and adds any “missing” keywords to the library that you just opened. In other words, all of your keywords will end up in all of your libraries.
On the one hand, this will ensure that you’ll have a consistent keyword structure for all of your libraries. On the other hand, if you’re trying to keep, say, work libraries separate from personal libraries, this feature could get to be a little annoying. There is a work-around, but it rather defeats the purpose of convenient library switching.
If you want to maintain separate keywords for each library, you need to delete two files from the Home > Library > Application Support > Aperture folder: keywordsets.plist, and keywords.plist. These are the “global” keyword databases that contain a running list of all of the keywords you’ve entered in any library. Each Library package also has its own plist that contains only the keywords that are relevant to it. So, when you delete the plists in your Application Support folder, Aperture uses only the keywords contained in the Library package that you’re currently using.