Boy.. that is going back... from memory, a library was used to basically save the state of web items so that they could be reused by other web templates .eg. you can spend a considerable amount of time configuring charts to look exactly how you want it to look. Once this was done, you could save these specific charts into a "library" and it was available in the WAD under the library tab (under the web item pane on left). So, if you launch WAD 3.x, click on the Library tab and see what's there...
I believe this feature disappears in WAD 7.x and is replaced by the "Reusable web items" tab, thus the need to migrate them first. There are ABAP reports you can run : RSZW_ITEM_MIGRATION_3X_TO_70 (using simulation mode) that should tell you if you need to clean this up.