Theory of Elastic Waves
This is a free, online physics textbook. According to the site, "For students with some background in mathematics and theoretical physics this textbook provides the basic knowledge of the theory of seismic waves which is necessary for the study of special monographs and scientific journals on this topic. Owing to the introductory character, theory and methods are explained by simple models to reduce computational complexity. These simplifications may hide occasionally the direct connection to seismology but there seems to be no other way if the aim is to provide analytical insight into wave propagation phenomena."