As was just mentioned, never underestimate the value of industry contacts - I suggest sending out some carefully-worded emails around to different companies.
Also the ESA (Entertainment Software Association) is a good place to start. They make a yearly report on games stats, I don't know if these links still work but I used them in my dissertation:
http://www.theesa.com/facts/pdfs/ESA_EF_2006.pdf
and
http://www.theesa.com/facts/pdfs/ESA_EF_2008.pdf
I found that their graphs showed a upward trend for adventure games in the video game market - in 2005 the genre didn't feature in the super-genres pie chart but in 2007 it takes a 4.3% chunk. Go check it out for yourself.
Hope this helps!