Complex sentence: Wollstonecraft was prompted to publish "Letters Written in Sweden" because she was in a great deal of debt and the successful sales of this, her most popular book in the 1790s, came at an opportune moment.
Split into simpler sentences: Wollstonecraft was prompted to publish "Letters Written in Sweden" because she was heavily in debt. The successful sales of this, her most popular book in the 1790s, came at an opportune moment.