Complex sentence: Boni was employed in an office before joining Liveright in the publishing business, had an idea to publish reprints of European writers, and in 1917 the company started the publication of the Modern Library series, which they later sold to an employee, Bennett Cerf.
Split into simpler sentences: Boni was employed in an office, but had an idea to publish reprints of European writers. The company started the publication of the Modern Library series in 1917, which they later sold to an employee, Bennett Cerf.