Can I find Fermat's complete works anywhere? I admire the mathematician very much and want to look at his writings.  Is there anywhere in book or web form that has a collection of his writings?
 A: You can find the letters  here.
A: The collected works of Fermat were published by Tannery and Henry, taken from http://www.fermat-science.com/pierre-fermat/ses-oeuvres/ here are the details:

[H.T. 1891], Oeuvres de Fermat, t. I, Œuvres mathématiques diverses – Observations sur Diophante, éd. P. Tannery et C. Henry, Paris, Gauthier-Villars, 1891.
[H.T. 1894], Oeuvres de Fermat, t. II, Correspondance, éd. P. Tannery et C. Henry, Paris, Gauthier-Villars, 1894.
[H.T. 1896], Oeuvres de Fermat, t. III, Traductions des écrits latins de Fermat ; de l’Inventum novum de J. de Billy ; du Commercium epistolicum de Wallis par P. Tannery, éd. P. Tannery et C. Henry, Paris, Gauthier-Villars, 1896.
[H.T. 1912], Oeuvres de Fermat, t. IV, Compléments par P. Tannery, éd. P. Tannery et C. Henry, Paris, Gauthier-Villars, 1912.
[H.T. 1922], Oeuvres de Fermat, supp. T. I-IV par M. C. de Waard, éd. P. Tannery et C. Henry, Paris, Gauthier-Villars, 1922.

The books seem out of print, but free (and given the age of the book likely even legal) can be found on the web, for example on the Internet Archive: archive.org.
