A fascinating book by a US professor of food psychology, Brian Wansink. It's not pitched as a weight loss (or gaining) book but has tips and ideas for cutting back about 100-200 calories a day -- that's the margin your brain doesn't figure it's missing out on.
It's interesting pretty much to everyone but I've had it lying around for about 2 years and it's really sunk in (and I've lost a stone in that time without trying or dieting -- lost another half stone, erm, getting pregnant and can't put it on which troubles me a little, but that's morning sickness for you)
Just really interesting experiments that show what is really going on when we buy and order and cook food. It's also got a chapter on children's eating.
A really good book that's not a diet book but which could help you lose weight (or gain it if that's your problem)