Some interesting on-line papers and books:
Excellent texts that unfortunately are not available on-line:
I did my PhD thesis on a language for "skeletons of proofs"; more precisely, on finding and expressing the skeletons of certain Category Theory / Type Theory proofs. Here are some links to people doing work that is somehow related to mine, but using approaches that are totally different:
My old math page - needs LOTS of updates
http://www.cs.bham.ac.uk/~pbl/ - call-by-push-value
http://www.math.mcgill.ca/triples/
Grothendieck: