Additional notes: Two mottled boards spiralbound on all four sides. There are seemingly no contents.
The title appears in cursive on a decal pasted onto the front cover.
The only other text is "ECHT 10=" at the top right of the front cover which appears to be the price.
Here is the official TZ description for that mysterious book you bought on Saturday:
(unknown artist) DOSSIER K. n.p. (Netherlands?), n.d. (1970s? 80s?). A spiral-bound notebook (9.5x7 in./24x18 cm) with a twist: the spiral goes all the way around, the covers can’t be opened, the contents can only be guessed at. The colophon (if any) is inside; besides a sticker with the book’s title on the front, the only other identifying mark is a small price tag which reads "10.- ECHT".