Library is a persistent storage for groups of puzzles for re-use in multiple projects. You can add a puzzle group to the library by right-clicking on it and selecting the option Save N Puzzles to Library. In the modal window type the name for the group as it will appear in the library's list of entries (refresh it by reloading the Puzzle editor).
A saved group of puzzle can be retrieved from the library by dragging it out to a workspace.
An entry can be deleted from the library by clicking Delete this item button (refresh the list of entry by reloading the Puzzle editor).