Complex sentence: Barbara Grier took over as editor in 1968, having previously contributed to the magazine under a variety of pseudonyms that included Gene Damon, Lennox Strong, and Vern Niven (making her most significant contribution as a book reviewer), seeking to turn it more professional.
Split into simpler sentences: Barbara Grier took over as editor in 1968, having previously contributed to the magazine under a variety of pseudonyms that included Gene Damon, Lennox Strong, and Vern Nive. She made her most significant contribution as a book reviewer, and when she became the editor sought to turn it more professional.