I was delighted to learn that the Dictionary of Medieval Latin from British Sources has finally been completed, a century and a year after the great project was launched. I don't suppose it will be joining my reference collection, but I love the fact that there are still such glacially slow academic projects coming to their belated ends even in these high-speed digitised times.
When it comes to Latin, sales too can be equally slow. The record is held by David Wilkins' translation of the Coptic New Testament into Latin. Published by the Oxford University Press in 1716 in an edition of 500 copies, it remained in print until 1907. I guess they didn't sell off remainders in those days.