I also very much recommend "The Art of Doing Science and Engineering", which is mostly philosophical but has quite a few partial differential equations in it.

I also very much recommend "The Art of Doing Science and Engineering", which is mostly philosophical but has quite a few partial differential equations in it. Available as a complete pdf here: http://worrydream.com/refs/Hamming-TheArtOfDoingScienceAndEngineering.pdf http://worrydream.com/refs/Hamming-TheArtOfDoingScienceAndEn...