I`m wondering if sombody could help me out with this abbreviation:
n.d.
as in the sentence: Eastern Counties Newspapers, n.d.:7. Clearly some reference, but what does n.d. stand for?

Without further context, and just based on
what information you give, this sounds like
it means "non-daily," that is, a newspaper
that is not printed daily, but rather:
weekly, or twice a week, etc. So, this
seems to mean that there are seven non-daily
newspapers in the eastern counties that
are being referred to.

maybe "no date"? Doesn't sound very logical..

Let me quote the whole sentence: In their 'Guide for Journalists', Eastern Counties Newspapers(ECN, n.d.: 7) proclaim:'Our papers are independent of all political parties and must be seen to be impartial and unbiased.'
Eastern Counties Newspapers is a news group in England, the quote is about their editorial policy. "n.d." refers to some kind of document issued by the group for its staff. This is not to be understood separately, it clearly refers to ALL the newspapers in the group. The number could refer to the page, but why n.d.? Beats me completely.

With this additional information, and without
further context, this still seems to mean
"non-daily." That is, ECN is a group of
seven newspapers, all of them non-dailies.
(If a newspaper is truly impartial and
unbiased and independent of political
influences, then it would have to be one
of the smaller non-dailies. The big
daily newspapers are all heavily biased
and politically influenced.)

"no date"? Well, it DOES make sense!
I`ve just checked their website: they publish four daily newspapers.

It's difficult to tell without the full context
in which this appears. "No date," or "not
dated," is a possibility, but from what
information you've given, this doesn't
seem to fit the context that well.
"New(s) document" might be possible:
a reference to a word processor document
file on which this was originally typed,
with the identifying number 7; or this
could be the approximate UK equivalent of
the US term, "internal memo," a
publication (identified by the number
7) for use within the organization
itself, on which a draft of this
advertisment originally appeared; or this
might identify this as an ECN non-news
publication (books, pamphlets, etc.), which
has the identifying number 7.

Thanks a lot Gyro. It does indeed seem to refer to some kind of memo (that`s what I first thought), but the number most likely refers to the page number. The other quote in the book with 'n.d.'is as follows: "[...] A good story does not need to be dressed up or overwritten. The facts will speak for themselves.(ECN, n.d.:13)."
This clearly indicates we are dealing with one document whose page (part?, issue?) numbers are given.
Surely, it would be best to get the explanation from the horse`s mouth, the publisher, but they don`t bother to reply to queries.

If this is a book, is there some kind of
bibliography somewhere in it with a list
of references? If so, then these sound sort of
like footnote-type references to a bibliography--
or something similar--with "n.d." being
the initials of someone whose work is cited.
And then the numbers would be page numbers.
Or, if the items in the bibliography are listed
numerically, then this would identify the
item in the list--for example, if the book
itself were written by someone with the
initials "n.d.", then somewhere in the book
there might be a list of articles or publications
written by him, and then the numbers would
refer to numbered items in that list.

If the "n.d.'s" only occur in association
with quotations, then this almost surely
marks them as footnotes to some type of
bibliography. Is there a name with the
initials "n.d." anywhere in the book? And
if so, is there a list of articles or
publications somewhere? Then
there would be some kind of correlation
between that name and the list.

I`ve just got the reply from the publisher. "n.d." stands for NO DATE.