I'll put this here where the foodies hang out. My criteria for a good cook book are quick(ish), reasonably straightforward, delicious, fail safe. In no particular order:
An Indian Housewife's Recipe Book - Laxmi Khurana
Quick, simple authentic curries with no frills or pretention. My copy is almost falling apart with use.
Essentials Of Classic Italian Cooking - Marcella Hazan
Throw away all your other Italian recipe books, this is the only one you'll need.
500 Low Carb Recipes - Dana Carpenter
Imaginative, cheap and delicious without being preachy or weird.
Nigella Express - Nigella Lawson of course
All her books are great but this is the one I use most often. Quick, tasty food.
The Book Of Jewish Food - Claudia Roden
Who says kosher food has to be bland and boring. Great recipes from all over the world mixed with fascinating history of the Ashkenazi and Sephardic communities and their food traditions.
I've got loads more cook books I love of course, but these 5 would be the ones I'd hate to lose.