I've never seen Holstein so I cannot comment. Groneman I find interesting from a historical perspective.
For somebody with an interest in the culture of the keris I believe that the best book to begin with is Visible and Invisible Realms-Margaret J.Wiener.
For somebody more interested in the physical entity itself, The World of the Javanese Keris-Garrett and Bronwen Solyom, and The Kris-An Earthly Approach to a Cosmic Symbol-David van Duuren.
For somebody who wishes to explore the fields of knowledge which embrace the keris:- Krisses-A critical Bibliography-David van Duuren.
Here is a link to a reading list that is perhaps as good as anywhere to begin the accumulation of printed matter dealing with the keris:-
http://www.kerisattosanaji.com/kerisinformation.html
you will find publishing details of the books I mention in this list.