Yes, Windows Neptune was a planned successor to Windows 2000, planned to be Windows NT version 5.5.
And Neptune had a successor of its own planned, Odyssey, planned to be version 6.0.
Just read the Wikipedia articles, and the sources to Paul Thurrott's stuff they provide. They prove that quite quickly.
However, Norman is failing to say the whole truth, namely, that later, Neptune, and Odyssey were merged into one project, Neptune becoming the Home/Personal Edition, and Odyssey the Professional, and Server, counterpart.