Is Clare W. Graves a credible author? I stumbled upon the book "The never ending quest" by Clare W. Graves. It looks interesting to me, but I suspect that he is not as scientific as he pretends to be. Is this author's theory of hierarchy considered credible?
Yes.  Clare W. Graves was a contemporary of Maslow.  He and Maslow used to go back and forth – as Graves thought Maslow's theory was incomplete (too closed).  At the end, Maslow agreed with Graves.  Graves died before much of his work was really accepted.  Cowan and Beck carried on his work.

It is, in my humble opinion, the most brilliant explanation for human development ever created.  It covers humanity as individuals and as collections (cultures), and Graves also created a system of change theory that matched.

You'll find the most ardent advocate is Don Beck, who was largely responsible (using the theories developed by Graves) to assist Mandela in South Africa.  It's called Spiral Dynamics Integral now by Don Beck.  There are many who continue studying this work.

Research confirms it.  There is ongoing research (Online PeopleScan) that is used by cutting-edge consultants in business and organizations.