I found the following on his website, but I don't have a date to accompany this comment. It also refers to his "latest novel, THE DEPTHS OF TIME was published at the end of February, 2000", so may not be very up to date.
I receive many queries, asking if I
plan to do any further work in the
series of books started with THE RING
OF CHARON and THE SHATTERED SPHERE.
The answer is yes -- but not just yet.
I have not decide when I will write
the next one, but I definitely will
write it. I have always intended to
write an open-ended series of book in
this universe. I plan to call the next
one THE FALLING WORLD. I looked over
my notes for that book a while back,
and I must admit reading them made me
want to get started on it soon.
However -- I do have other books I
want to write as well. Sooner or
later, I will juggle my time enough to
let me make a start on THE FALLING
WORLD. I'll keep you posted.