This is one of those esoteric old textbooks that no one has bothered to update for modern engineering students. It's considered to be the classic continuum mechanics because you will have had to take all the basic Mechanical Engineering courses, (Lin Alg, Calc, Heat transfer, Mechanics, Fluid Mechanics) before reading it. Pretty wild.
For any serious student of continuum mechanics, this is an excellent book covering the mathematical concepts necessary for a rigorous treatment of the subject.