(Sorry if this has been posted before or is widely known; I haven’t found any reference anywhere.)
When your script library starts to grow, you may want to start keeping your scripts organised into subfolders of the Script Libraries folder. Fortunately, this is possible! To load a script library, say, in ~/Library/Script Libraries/My/Nested/Lib/SomeLib.scptd, just specify its (Unix-style) relative path:
use SomeLib : use script "My/Nested/Lib/SomeLib"
This works even when SomeLib is inside a script bundle (as opposed to the user or system-wide Script Libraries folder).
This feature enables developers to distribute several related library scripts within a single folder, and helps avoid conflicts between scripts with the same name but from different developers (just keep them in different subfolders).