Mathematics equivalent of Feynman's Lectures in Physics? I'm looking for an equivalent of "Feynman's Lectures in Physics" in mathematics. I'm specifically looking for book/books that delve into, using Feynman's words, "the meaning of things".
 A: since you ask specifically about "the meaning of things", this might be what you are looking for:
What Is Mathematics? An Elementary Approach to Ideas and Methods

For more than two thousand years a familiarity with mathematics has
  been regarded as an indispensable part of the intellectual equipment
  of every cultured person. Today, unfortunately, the traditional place
  of mathematics in education is in grave danger. The teaching and
  learning of mathematics has degenerated into the realm of rote
  memorization, the outcome of which leads to satisfactory formal
  ability but does not lead to real understanding or to greater
  intellectual independence. This new edition of Richard Courant's and
  Herbert Robbins's classic work seeks to address this problem. Its goal
  is to put the meaning back into mathematics.
Formal mathematics is like spelling and grammar--a matter of the
  correct application of local rules. Meaningful mathematics is like
  journalism--it tells an interesting story. But unlike some journalism,
  the story has to be true. The best mathematics is like literature--it
  brings a story to life before your eyes and involves you in it,
  intellectually and emotionally. What is Mathematics is like a fine
  piece of literature--it opens a window onto the world of mathematics
  for anyone interested to view.

