Addendum to original comments:
As I start to collect increasingly rare and unusual L√ľtken pieces, I'm finding this book progressively more useful. Many 'unika' items are signed with L√ľtken's own four-digit numbering system, and this book sorts all the designs by said system, so it has been invaluable
for attributing many dates of design over the last year and a bit.
It's still mostly useless to the general collector/dealer, though. I stand by that comment.