with Boundary Value Problems (W.F. Trench)
Elementary Differential Equations with Boundary Value Problems by William F. Trench is a free open textbook covering first order equations, numerical methods, linear second order and higher order equations, series solutions, Laplace transforms, linear systems, Fourier series and boundary value problems.
This is the web edition of the free open textbook by William F. Trench (Trinity University), rendered from the author's own LaTeX source. It is licensed Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported; the author's edition of record is Free Edition 1.01 (December 2013). Chapter, section, equation, theorem, example and exercise numbers match that edition exactly.