This is a very rare and nearly never found 1784 Sackbuch Calculator Book.  Book is leather bound with heavy parchment paper providing calculating directions and many tables to look up answers.  It is all in German.  In the 1700s, Bankers, Mathematicians and Merchants would carry this calculator booklet in their pocket to barter or calculate goods.  Book measures 4” x 8” with two leather straps.  The word Sackbuch is German and is interpreted as “Sack” as a sack or bag and the German word “Buch” as a book.   For over 223 years of calculating computer age, leather book shows slight wear, but has preserved parchment pages in very good condition including binding.  It is truly the Holy Grail to calculating computer instruments of the period.  Only two others are known to exist.  We have this one and the other is with a private German collector.  This is truly a very rare treasure that will highlight and accent any collection of early calculators.