thanks to all for the reply.
my school has indeed a library where we had a look some time ago.
there's a whole rack full of books on RF design, but most of them that
cover stripline or microstrip are bulky things full of formulas we
don't need.
we're not really looking for such thing, we are looking for a way to
solve the problem with pencil and caculator, esprecially for the
interdigital filter cos we have no clue about that.

so far we started from a 3th order filter with lumped elements and
then converted it to a structure with kuroda elements.
now what we need to find out is how to calculate the dimensions of the
coupled line filter and the interdigital filter, given the kuroda
filter structure. we already managed to do this for the filter with
stubs (the most easy one probably :-)