Complex sentence: Myers Dennis and a friend marketed the book to the booksellers at the height of its popularity it was selling more than 1,000 copies a day; throughout 1955 and 1956 it sold between 1,000 and 5,000 a week.
Split into simpler sentences: Myers Dennis and a friend marketed the book to the booksellers. At the height of its popularity, it was selling more than 1,000 copies a day; throughout 1955 and 1956, it sold between 1,000 and 5,000 a week.