Just discovered that if you’re using the desktop client (at least the one on Mac, haven’t tested in Windows yet), you can create links to other notebooks. Select the note you want to link to. From the Note menu, select ‘Copy Note Link’.
Now goto the Note you want the link in. Paste the link and it should appear highlighted and underlined. Clicking this link will open the note. The pasted link also appears to work correctly in the Web App even though I haven’t found a way to create them using the Web.