Tintin Forums

They are an edition of the albums issued on subscription in France and Belgium, which, in the manner of the old Making of books, or the current series of Young Readers, have background information on the genesis and production of the albums, and give context to the work of Hergé and his studio.
You'll find a thread on them here.

Actually, there is also another official version which was published in 1979 by Brabantia Nostra: the "Kuifje-index". I have this one at home, bought it like 20 years ago for 1 euro tops. The story was published in French although the edition itself is Dutch.

Sorry, the Kuifje Index isn't an official publication; as far as can be told it was fan produced, and the reproduction of the Detectives strip was most probably a pirate (can't say for certain, as I have nothing on whether or not they came to some arrangement with someone to print it).

It was an official release. Having said that, I will check with some people who know for sure.

The Brabantia Nostra publication was a reprint of the 1976 CISO (Comics Information Service Organization) issue of the same name (Kuifje Index), which as far as I know, was a Flemish fanzine produced unofficially, and one of their aims was to gather odds and ends of strips which hadn't been gathered elsewhere. I rather took it that their activities were at best tolerated rather than authorized, but I'm happy to be made better informed...! :-)

Update: A bit more digging: it compiles articles from several issues of CISO, rather than reprinting just one.

This is a complete revelation to me.
Very interesting thread /threads.