I've got three cookbooks I turn to all the time for fairly quick weeknight meals:
1) America's Test Kitchen Quick Family Cookbook (there's a newer edition but I haven't seen it) Variety of recipes nicely organized into chapters like "Eggs for dinner" or "Dinner off the grill" and in a very handy spiral binder. My 10-year old happily eats anything from this.
2) Vegetables Every Day: The Definitive Guide to Buying and Cooking Today's Produce I use this at least once a week. This is a very practical vegetable A-Z cookbook that has recipes for both side dishes and main dishes for most common vegetables. Many are quite simple and quick - the arugula salads in here are some of my go-to super fast salads and I love his asparagus recipes.
3) The Perfect Recipe for Losing Weight and Feeling Great Much better than it sounds from the title! A lot of the book is mix-and-match kinds of recipes - formulas like one pound of protein with a list of recommendations plus x amount of vegetables, mixed with a spice rub or sauce with a list of different flavors to choose from. Almost everything is pretty easy and there are a lot of good ideas for how to take some lean protein and mix it with vegetables and interesting seasonings to make a good, healthy meal. I especially like the crustless quiche ideas and the foil packets that can be baked or grilled.