I guess you'll find what you need in the monograph by J.V. Field and J.J. Gray, <A HREF="http://journals.cambridge.org/abstract_S0007087400024444">The Geometrical Work of Girard Desargues</A>.

> They give substantial critical and
> exegetical commentaries as well as
> valuable introductory essays placing
> Desargues and his various works in
> historical context. Particularly
> attractive is the authors'
> demonstration of how Desargues' highly
> original ideas developed from the
> contemporary technical mathematical
> context, and their elegant exposition
> of the importance of the 'practical
> tradition of applied geometry' in
> stimulating his work.

Desargues had a limited range of interactions, which apparently did not include Kepler. Quoting from <A HREF="http://www.bookrags.com/research/girard-desargues-and-projective-geo-scit-03123/">Gale's Science and Its Times</A>:

> Desargues spent
> many years in Paris with a group of
> mathematicians that included Descartes
> and Pascal as well as the Jesuit
> scientist Marin Mersenne (1588-1648)
> and Etienne Pascal (1588-1651).
> Desargues's work on projective
> geometry was printed principally for
> this limited readership of friends.
> Unfortunately, however, his views were
> very unorthodox and unpopular during
> his life — Blaise Pascal (1623-1662)
> was one of his few admirers. Only 50
> copies of his book on projective
> geometry were printed, many of them
> later destroyed by the publisher.
> Desargues's work slipped into
> obscurity for nearly 200 years after
> the publication of his defining text
> on the subject.

One piece of trivia, on the origin of the concept of *involution* caught my attention:

> Desargues introduced more than 70 new
> terms in his book, of which only one,
> **involution**, has survived. This term,
> which denotes quite literally the
> twisted state of young leaves, is used
> to designate the projective
> transformation of a line that
> coincides with its inverse. (Most of
> the terms that Desargues proposed were
> based on obscure botanical
> references.)