Complex sentence: His next major work was his unique "Bibliotheca" (1545), a landmark in the history of bibliography in which he set out to catalogue all the writers who had ever lived and their works.
Split into simpler sentences: His next major work was to catalogue all existing publications, his "Bibliotheca" of 1545, a landmark in the history of bibliography. In 1545 he published his unique "Bibliotheca universalis", an attempt to catalogue all the writers who had ever lived and their works.