Documents on the GHC Core Language
http://www.haskell.org/ghc/documentation.html
has a link to the September 2001 (Draft for GHC 5.02) document
describing GHC Core (in what is for me user-hostile .ps.gz format.)
And this page
http://www.haskell.org/ghc/docs/latest/html/users_guide/ext-core.html
promises an easier format PDF document, but the link is broken.
I did eventually find the 1st April 2009 GHC 6.10 document here
http://www.haskell.org/ghc/docs/6.10.2/html/ext-core/core.pdf
and a bit on this page
http://www.haskell.org/ghc/docs/latest/html/users_guide/options-debugging.html#id468571
about GHC core. I haven't read these yet, but could I ask whether they
constitute the complete current documentation for GHC core? (I'm just
curious to get a flavour of what core does.)
There's also the commentary page:
http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/CoreSynType"