I'm interested in finding a free online textbook that is an introductory textbook on numerical PDE for engineers. It should cover the basics of finite differences, finite elements, and Fourier series methods for very simple PDE cases, mainly the 1D heat and 1D wave equations, and should not be concerned with rigor, proofs of convergence, etc.
The purpose of this request is to find a decent online resource for students to use as an alternative to an official course textbook. The students are undergraduate engineers who have seen multivariate calculus and linear algebra but no other mathematics.
Does anybody have suggestions?