I'd recommend A Mathematician's Apology for a defense of number theory by someone who was quite prominent in the field. It's slightly dated ("No one has yet dis

I'd recommend A Mathematician's Apology for a defense of number theory by someone who was quite prominent in the field. It's slightly dated ("No one has yet discovered any warlike purpose to be served by the theory of numbers or relativity, and it seems unlikely that anyone will do so for many years."), but a lot of it is still relevant, or at very least, interesting.