chunk: The poem is separated into twelve "books" or sections, the lengths of which vary greatly (the longest is Book IX, with 1,189 lines, and the shortest Book VII, with 640). The Arguments at the head of each book were added in subsequent imprints of the first edition. Originally published in ten books, a fully "Revised and Augmented" edition reorganized into twelve books was issued in 1674, and this is the edition generally used today.
  The poem follows the epic tradition of starting in medias res (Latin for in the midst of things), the background story being recounted later.