The book is ?A la découverte des souterrains de Paris? by Patrick Saletta.
As you know, the Guardian report is dated 8 September, 2004. The
relevant extract is,
"Patrick Alk, a photographer who has published a book on the urban
underground exploration movement and claims to be close to the group,
told RTL radio the cavern's discovery was "a shame, but not the end of
the world". There were "a dozen more where that one came from," he
A similar article appears on the French RTL web site dated 3
September. Part of the final paragraph reads,
"Pas grave, "il y en a dix autres dans les galeries", explique un
proche du groupe, Patrick Alk, photographe, adepte des sous-sols
depuis les années 80 et auteur d'un livre intitulé "A la découverte
des souterrains de Paris"."
Google translates this as (sorry, French is not a language of mine),
"Not engraves, "it there has ten others in the galleries", explains of
them a close relation of the group, Patrick Alk, photographer,
follower of the basements since the Eighties and author of a book
entitled "A the discovery of the undergrounds of Paris"."
So his book is: A la découverte des souterrains de Paris.
A search on that title reveals that it is written by Patrick Saletta.
"Title : A la découverte des souterrains de Paris
Authors : Patrick Saletta
Publisher : SIDES
Year : 1990
ISBN : 2868610757
Language : French
Description : A global and exhaustive survey of the City of Paris
underground network (catacombes, quarries, churches, metro, Telecom,
sewers, etc.) by a very famous urbangrounder who became one of the
major French specialists on underground.
This very complete book was an event at the time of the publication
and is still a best seller.
Lots of photographies, maps, plans."
The book is available through amazon.fr
Other references to the book can be found by this search strategy
"A la découverte des souterrains de Paris"
I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.
Search strategy: as described above