I have recently bought two new books, further ridiculing my old claim that there almost no books on the administrative system. Both are anthologies of scientific papers by various scholar of Thai studies, and it's hardly surprising that each contains one by late David K. Wyatt, maybe the most prolific in this field. As a side note, Rikker had a review of Professor Wyatts Southeast Asian library on his blog.
The first book is Regions and national integration in Thailand, 1892-1992 compiled by Volker Grabowsky. It is the compilation of papers presented at a conference at the University of Passau. As one can see from the title, this conference already took place in 1992, and the book was published in 1995, but it is still on sale by the publisher. The conference did take place at the centennial of Prince Damrongs appointment as first Interior Minister and the begin of thesaphiban reforms.
Much older is the second book, Modern Thai politics : from village to nation published in 1976 and compiled by Clark D. Neher. I found this one at a bookseller in the US selling it for 1$ (however shipping over the atlantic was another 8$), and it is a somewhat torn former library book. It was outlisted by the Loyola library in Chicago and now gets its place in my personal library.
Actually reading them have to wait till I return from Thailand, and hopefully I will find more interesting book there to further enlarge my library. But with just 90 books on Thai topics I doubt I'd ever get close to the 15,000 books Professor Wyatt collected.