History of Mathematics
Some primary source material.
  • Euclid's Elements (c. -300), selections from the beginning and end.
    Note his treatment of Eudoxus-style arguments on area versus radius.

  • Russell and Whitehead's Principia Mathematica (1910), a selection on the number one.
    Check out how formalized the mathematics has become, and compare that with the kinds of statements they are trying to prove-- look in particular at the middle of page 362.

  • Bourbaki's Elements, from the volume on Topology (1940), a selection on the construction of the reals and part of the "historical note" .
    Here the math should be much more readable, but I mainly just want you to get a sense of their style.