The linear polaron model is an excellent example of an exactly soluble, yet nontrivial polaron system. It serves as a trial system or zero-level approximation in many sophisticated methods of polaron investigation. This book analyzes, in particular, the possibility of reduction of the full polaron Hamiltonian to the linear one, and introduces a special method of calculating thermodynamical characteristics based on the calculation of the averages of T-products. This T-product formalism seems to be a more convenient way of doing similar calculations involving Feynman's path integral approach.This book follows a step-by-step approach, from comparatively simple physical ideas to a clear understanding of sophisticated mathematical tools of investigation in modern polaron physics. The reader is able to compare the physical point of view with methods proposed in the book, and at the same time grasp the underlying mathematics.Some familiarity with quantum statistical mechanics is desirable in reading this book.