I finished reading Difficult Questions About Videogames last week, and can thoroughly recommend it: it's a lovely dip-innable book cataloguing the answers of c.70 folks from the games industry, academia and elsewhere, to questions like "What is a videogame?" and "Why is playing videogames fun?".
"In one sentence, games are fun because they allow us to rehearse basic survival skills and we are hard-wired through evolution to feel good when we do that." - Noah Falstein
"People are pattern-matching machines and we find pleasure in patterns." - David Thomas
"What a silly thing it is that we insist on propagating that epithet anyway, 'gamers'. Do you ever hear Hollywood targeting a film for 'hard-core moviegoers'? Or a publisher talking about a new book that will really get the 'bookreaders' talking?" - Ian Bogost
Suzie Cardwell also had some nice thoughts on casual gaming, prompting me to google and find her powerpoint presentation on women and casual gaming - well worth a look.