Business letters by the Ari? Cool. I suppose he must have done something for a living, because being a mystic doesn't pay, and he couldn't have made money from his writing because he didn't do any (all we know today of his teachings comes from R Chaim Vital's private notes, which weren't written for publication - the manuscript was stolen and copied without his permission). Did the man tell you what business the Ari was in, at least as reflected in those letters?
Re Jastrow, I must admit I never read the preface either. At my yeshivah the use of Jastrow was very mildly discouraged. We were supposed first to try looking things up in the Aruch (an 11th-century Aramaic-to-Hebrew dictionary compiled by R Nathan ben Yechiel of Rome), and only if we couldn't find it there, or couldn't understand the definition, were we to have resort to the "easy" way of Jastrow. Hebrew words were also first to be looked up in Even-Shoshan (a Hebrew-to-Hebrew dictionary), and only afterwards in a Hebrew-to-English dictionary. (We were allowed to use Steinsaltz only to look up incidental references, not the main text we were supposed to be studying; the volume for that tractate was removed from the library. Otzar Meforshei Hatalmud ("Treasury of Talmud Commentaries") was banned, being described as "spoon-fed rishonim". Mei Menuchot (a very thorough commentary on the Tosefot) was allowed only for the beginners' class.)
no subject
Re Jastrow, I must admit I never read the preface either. At my yeshivah the use of Jastrow was very mildly discouraged. We were supposed first to try looking things up in the Aruch (an 11th-century Aramaic-to-Hebrew dictionary compiled by R Nathan ben Yechiel of Rome), and only if we couldn't find it there, or couldn't understand the definition, were we to have resort to the "easy" way of Jastrow. Hebrew words were also first to be looked up in Even-Shoshan (a Hebrew-to-Hebrew dictionary), and only afterwards in a Hebrew-to-English dictionary. (We were allowed to use Steinsaltz only to look up incidental references, not the main text we were supposed to be studying; the volume for that tractate was removed from the library. Otzar Meforshei Hatalmud ("Treasury of Talmud Commentaries") was banned, being described as "spoon-fed rishonim". Mei Menuchot (a very thorough commentary on the Tosefot) was allowed only for the beginners' class.)