Many secular Sonlight users keep Hillyer's A Child's History of the World as the spine for their history lessons. It isn't overly religious but I found it to be slightly condescending and at times bigoted towards non-christian cultures. We replaced it with Susan Wise Bauer's The Story of the World, volumes 1 and 2. If you decide to go with The Story of the World check out this website which matches up the chapters of A Child's History of the world with The Story of the World. It makes connecting the books to the plan in your instructor's guide much easier.
Spragget and Johnstone's Window on the World has many pages of ways to pray for non-Christian peoples and describes many different countries in regard to their progress toward Christianity. We replaced it with DK publishing's A Life Like Mine and First Atlas.
Read Alouds and Readers
These are the books we left out and are really the easiest to replace. If it was a biography/memoir I tried to replace it with a biography/memoir. If it was fiction/literature I replaced it with a good children's literature title. Have fun and choose what you and your children will enjoy.
Titles we replaced:
And the Word Came With Power by by Joanne Shetler, Patricia Purvis
More Stories from Grandma's Attic by Arleta Richardson