Concern over plagiarism in designing online course
So, as a matter of fine distinction, what you are doing cannot be plagiarism if it is credited.  It could still be theft, and it could still be unethical, but plagiarism is passing someone else's work off as your own.
If you are having the students purchase the textbook, I would reach out directly to the author and explain what you are doing.  Since he will be compensated by the textbook purchases, he may well agree to allow it, which would eliminate the ethical issue.
Even if you are not having students purchase the book, some authors are simply generous enough in spirit that they would nevertheless agree to such an arrangement.
The ethical alternative is to come up with your own exercises and explanatory texts.  These can closely follow the spirit and outline of the author's text if you think he has a great approach, and you can point students towards his book as an excellent and highly recommended source.  (You would still have to credit the author to the degree that his contributions and organization of the material is a resource for you, but if you do not use his text or exercises, then that would be all you would have to do.