Second vote for Madhur Jaffrey's World Vegetarian here! I have many others, but I often reach for that one and I've made more recipes from it than all my other veghead cookbooks.
EDIT: it occurs to me that Mark Bittman's got a pretty good vegetarian cookbook too... something like How to Cook Everything Vegetarian. I don't have it but I do want it.
My husband and I are long-time vegans (30+ and 15+ years). Our cookbook collection includes:
Simply Vegan by Debra Wasserman
The Vegan Epicure by Hermine Freed
Sinfully Vegan by Lois Dieterly
My Sweet Vegan by Hannah Kaminsky
Spicy Vegan by Sudha Raina
The Gluten-Free Vegan by Susan O'Brien
The Mediterranean Vegan Kitchen by Donna Klein
Everyday Vegan by Jeani-Rose Atchinson
I got my mother Veganomican last year for Christmas- she returned it and got "How to Cook Everything- Vegetarian" by Mark Bittman, said that there were more general recipes for everything in it. The original HTCE is really a fabulous all-purpose, and she's loved the veg version this year- I'd like a copy myself.