
When you copy an album or event from one library to another using iPhoto Library Manager, the destination library will make its own copies of any photos that are being transferred. If you no longer want those photos to be in the original library, you can safely delete them without it affecting the copies that are in the library you copied the album to.
One common task that is often needed when managing your libraries (especially when splitting a libary in two) is to delete an album along with all its photos from one of your libraries. This is tricky, since when you select photos in an album and press the "delete" key (or drag them to the trash), it does not remove the photos from your library as a whole, it only removes them from the album. And, of course, deleting the album itself also leaves the photos in your library. The trick is to select all the photos and hit command-option-delete. This will remove the photos from both the library and the album.
To entirely remove an album of photos from a library: