Complex sentence: This edition was heavily revised by Mary Shelley, partially because of pressure to make the story more conservative, and included a new, longer preface by her, presenting a somewhat embellished version of the genesis of the story.
Split into simpler sentences: This edition was heavily revised by Mary Shelley, partially because of pressure to make the story more conservative. It included a new, longer preface by herself, presenting a somewhat embellished version of the genesis of the story.