That is an interesting question on the worst numismatic book. In my opinion, none in my library would qualify. However there is one I will suggest. It is the Devonshire sale catalog of March, 1844.

I quote a passage from British Numismatic Auction Catalogues by Harrington E Manville. On page 85, "The Devonshire Catalogue is worthless, upwards of 400 serious errors have been counted in it. Many valuable coins are lotted as false which were probably genuine and sold in lots of 20 & 30 each for nothing --and as no proper opportunity was given to view them, in numerous cases the prices afford no criterion as to their value. It was rather a game of chance or a scramble than a sale."

WOW, what a description!