Complex sentence: By 1994, they wrote their first book, "The Beardstown Ladies' Common-Sense Investment Guide", which sold over 800,000 copies by 1998 and was a NY Times best seller.
Split into simpler sentences: By 1994, they wrote their first book, "The Beardstown Ladies' Common-Sense Investment Guide". Which sold over 800,000 copies by 1998 and was a NY Times best seller.