Differential Equations with Applications and Historical Notes, George F. Simmons, McGraw-Hill, 1972.

I acquired this from Amazon based on a recommendation from Steven Strogatz's (@stevenstrogatz) twitter feed on January 9.  Strogatz commented: My favorite preface to a math book is that written by George Simmons in "Differential Equations with Applications and Historical Notes." He then went to on to quote part of the preface.  It was so attractive that I had to see the book, so I ordered a copy.  I got a first edition copy, there are also second and third editions.  The book is well suited to physic and engineering students.  I could say more, but check it out in your library - it is well worth your time.

Introductory Eigenphysics, Clive A. Croxton, John Wiley & Sons, London, 1974

I recently acquired this is as a used book on Amazon, the forward was written in 1973, two years before I started university.  I wish I had found it long ago.  It is a unified treatment of the partial differential equations of physics.  Croxton's stated aim is to "attempt to break down the conventional divisions between quantum and classical mechanics, between physical optics and electron scattering, between electrostatics and aerodynamics, and so on . . .."  The author succeeds admirably.  If you have never seen the book, take a look at it unifies material across multiple different courses.  My copy was a withdrawn copy from the Cal Poly library.

The availability of this book begs the question: why did a library retire a book of enduring interest?  It is useful today as the day it was written.  Many years ago when the library at the University of Central Arkansas was culling its collection, I was the library representative for the Department of Physics.  They wanted to retire many books simply because they were old.  I only allowed them to cull some books they shouldn't have acquired in the first place (think vanity press publications such as a book on general relativity by a urologist as a typical example.)

Anyway, this book is a hidden gem - every bit as good as his famous text on liquid state physics.