but I did not use or have it so can't comment. There are more on amazon.
But I do not use books specific to Mathematica. I have general books on PDE's numerical solutions, which I can implement in Mathematica. I prefer to implement something myself than use a blackbox solver (unless I am just doing something quick to look at), since I learn more that way.
Instead of spending the time of learning how to use a blackbox solver with all its options and suboptions, I can spend the time learning the actual algorithm used to solve a PDE, which is more useful for me.
Even with Matlab, I do not use the pde toolbox, but wrote my own code. (But I am a student, so things might be different for others)