What is a secular textbook?
A secular home school curriculum textbook is a book written usually by a non-Christian, for the purpose of teaching academic subjects. Some of these books, such as those offered through the Calvert Correspondence Course, do an exceptional job of teaching.
A secular curriculum may vary widely in the presentation of secular values, and the older they are; the less likely they are to be offensive. "The Golden Rule Reading Series," published in the 1950s by the State of California, is full of wonderful character-building stories and some of these stories are from the Bible. These are the books that were reprinted by ABeka for their reading program.
Care needs to be taken, however, with math books written in the 1960s and 70s because "new math" was being pushed at that time, and proved to be a failure. A good math curriculum is Saxon Math.
If you are using a secular home school curriculum, you should pre-read everything your children are reading. This is especially true for the more subjective areas such as reading, social studies and science, and more so if the books were published after 1975. Even in math books, word problems may emphasize women in non-traditional roles and other off-the-wall propaganda that you wouldn't expect to find in a math book. These objectionable items do not necessarily mean you should toss out the books. They can be the basis for some lively discussion.
A well-educated Christian child should at some point be exposed to how the World thinks. This is best done in a controlled situation, such as a home school. The parent needs to bring this kind of thinking into the light of Scripture, and expose it as a lie. If you are not willing to go to this extra work, or do not know how to recognize humanistic teaching, do not use a secular home school curriculum with secular textbooks. Even if they are free, they are not worth it.
For guidelines on choosing curriculum go here:
What About Home School Curriculum?
For reviews of home school publishers go here:
What Home School Textbooks Should I Use?
For curriculum prices by grade level go here:
Learning Things - Home School Curriculum