Through my interaction with book clubs, I learned often clubs emphasize different types of books. Some specialize in historical, ethnic, female-only, biographies, non-fiction/fiction, political, social issues or other single-theme works.
However, I believe it broadens the readers if they sometimes go outside their normal themes. For instance, for those groups who may read only books by female authors, why not read a work by a male author and discuss how women are portrayed in another cultural context. (Memoirs of a Geisha, by Arthur Golden or A Thousand Splendid Suns by Khaled Hosseini).
For those groups that specialize in historical works, try reading the biography of an important person whose life impacted historical events about which the book is written. This gives an additional view of the era of the event. Biographies or autobiographies usually contain information about people that make for rich discussion of how specific historical events may be portrayed differently. An excellent autobiography, A Mirror to America by John Hope Franklin, is a great overview of the major events of the 20th Century, all of which he experienced. It is rich with detail by a man who kept his life moving forward, despite the challenges of his race.