
One of the primary uses of iPhoto is to import and organize photos you have taken with your digital camera. There are a couple things you should know about how this works when you have multiple iPhoto libraries.
If you have iPhoto set up to launch automatically as soon as your plug your camera in, this will cause iPhoto to open whichever library you had last chosen to work with in iPhoto. If you typically only import photos into a single one of your libraries, this can sometimes cause the wrong library to be opened up, at which point you have to quit iPhoto, go open up iPhoto Library Manager, switch to the correct library, and then open up iPhoto again. If this happens to you a lot, you might want to instead set iPhoto Library Manager to open up when you attach your camera rather than iPhoto. This way, you can be sure that you have the right library selected before opening up iPhoto, and avoid importing photos into the wrong library.
To set this up, open up the Image Capture applications located in your main Applications folder on your hard drive. If you're running OS X 10.6 or later, connect your camera to your computer, and it should appear in the "Devices" list on the left. In the lower left corner of the window, click the pop-up button and select "Other…" menu item. Find iPhoto Library Manager on your hard drive and select it to tell Image Capture to open it whenever you plug in that particular camera.
If you're running OS X 10.5 or earlier, open the preferences window, and select the "Other..." menu item from the pop-up menu labelled "When a camera is connected, open:". You'll be prompted to select an application, at which point you can select iPhoto Library Manager, and click OK. Now, when you plug in your digital camera, iPhoto Library Manager should automatically open instead of iPhoto.