It is absolutely necessary to read whole text book for SL course??

[Merlin]

It all depends on what you know.  This entire industry/corner of education is based on the idea that a person can prove knowledge of a subject by taking a test.  For years, students took CLEP tests and AP tests and these were generally accepted as evidence of knowledge in the subject area.  Most of the alternative credit providers use this general methodology.  If a student can achieve 70% on a test, then he/she knows the subject and gets the credits.  If you know a subject well, you don't need the book.  If you don't know a subject well, you need the book.  I have had courses in which I have needed the book and many in which I have not.

In addition to the above, it also depends on the course. Some of the courses have exams that cover specific sections of the book. Like there will be a midterm that covers the first half, and a final exam that covers the last half. In other cases, it will just be a final exam where you don't know what is being covered.

Personally, for the courses I took at SL, I read each of the books. Or at least, I used academic skimming and scanning techniques to try to absorb the crux of every section that was relevant to my test. For areas that I felt would be covered in more detail in the test, I used engaged reading to focus on those areas. If you're not familiar with skimming and scanning from a college textbook perspective, I suggest that you watch a few youtube videos on the subject, or google it for more details. It will save you a ton of time and keep you far more focused. It works very well in combination with proper college-style note-taking techniques as well.

Of course, many of the StraighterLine courses are open book and open notes, so if you've skimmed/scanned the book (and taken good notes) well enough to know where to find information quickly, you can use the online textbook during the exam to help you answer questions. You won't have time to look everything up, so while some recommend just using search and trying to avoid reading, in practice it doesn't work well. But using the textbook in coordination with a good understanding of the major topics will be a huge benefit.

That said, SL doesn't really offer much in the way of courses that you can't get from Study.com, and I found that Study.com provides a much better learning environment. They pare down the massive textbooks to the meat of each topic so you're not stuck reading through all the fluff that textbook authors add to increase the page count in order to justify the cost of the textbooks. Plus you get the benefit of both video and text lessons, which I find much preferred. So unless you're in danger of taking too many credits via Study.com, I recommend ignoring SL and just focusing your efforts on Study.com if you can.

That's my perspective at least.