Benjacob corresponded with Jewish scholars in Western countries, and was known during his lifetime for his great achievements as a bibliographer, although his monumental work, the "Otzar ha-Sefarim, Thesaurus Librorum Hebræorum tam Impressorum quam Manuscriptorum," did not appear till seventeen years after his death (Vilnius, 1880). It was published by his son Jacob, and contains 17,000 entries of Hebrew printed and manuscript works, with valuable notes by M. Steinschneider. An author-index to the work together with additions has been promised (as of 1906) by Steinschneider ("Hebr. Bibl." xx. 73; "Festschrift," p. vii.). It is the greatest Jewish bibliographical work in the Hebrew language, and is still the standard bibliography of printed books down to 1863.