I'm experimenting with add-ins to work like a library for VBA projects. This looks like a COM DLL written in Visual Basic.
Create a normal spreadsheet and write classes and or modules in it. This normal spreadsheet is just for testing and debugging. If there are no more bugs, you can remove everything on the worksheet. Unfortunately even this will be a library, it is required that there is at least 1 worksheet available. After this you can save your work as an add-in. So do so.
Now it is time to activate the add-in as a regular add-in like Analysis toolpack. Now you can create a normal spreadsheet and go to the VBA editor and add a reference to that add-in.