Elements Of Partial Differential Equations By Ian Sneddon.pdf ((full)) -

The first chapter is a deep dive into Pfaffian forms. Don't skip this; the rest of the book relies on you being comfortable with these foundations.

The book begins with an introduction to PDEs, including definitions, examples, and classification of PDEs. The author then discusses the wave equation, the diffusion equation, and Laplace's equation, which are three of the most important PDEs in physics. The first chapter is a deep dive into Pfaffian forms

The book was originally published by McGraw-Hill. Later, Dover Publications (known for reprinting classic math texts) released an inexpensive paperback edition. Dover is a legitimate, active publisher. the diffusion equation

If you cannot find a legitimate PDF of Sneddon and you need free, high-quality PDE resources, consider these : and Laplace's equation