Complex sentence: Her 1896 book "Lessons in Truth" is now considered the core text on Unity Church teachings, is the is the most widely read book in that movement, and has sold over 1.6 million copies since its first publication.
Split into simpler sentences: Her 1896 book "Lessons in Truth" is now considered the core text on Unity Church teachings. Is the is the most widely read book in that movement, and has sold over 1.6 million copies since its first publication.