You're in luck - seasonal, vegetarian, and local are all the rage in cookbooks right now. It seems like a new seasonal cookbook comes out every week.
Jack Bishop (of Cook's Illustrated) has a book called "A Year in a Vegetarian Kitchen: Easy Seasonal Dishes for Family and Friends." It is organized seasonally, and the dishes sound interesting: Curried Eggplant and Mango Stew, Penne with Broccoli & Pumpkin-Seed Pesto, Fettucine with Mascarpone & Sage-Walnut Butter. Sadly, this book doesn't have any desserts.
"Clean Start: Inspiring You to Eat Clean and Live Well" and "Clean Food: A Seasonal Guide to Eating Close to the Source", both by Terry Walters, are vegan and laid out seasonally. She focuses on avoiding packaged foods, balance of flavor and color in meals, and healthy habits.
"Vegetarian Cooking for Everyone" by Deborah Madison has been a favorite of mine for many years. The section on vegetables is organized alphabetically by veggie, and each section starts with What To Look For/How To Store/How to Use/Special Handling/Quantity, then gives several recipes for the veggie in question. The recipes are generally concise and straightforward, with few ingredients. Madison also has a book called "Local Flavors: Cooking and Eating from America's Farmers' Markets," which I haven't used, but I'd recommend it based on my success with her other books.
"Rustic Fruit Desserts" by Corey Schreiber & Julia Richardson is my go-to book for fruit. I've made about a dozen desserts from this book, and every one has been fantastic. The Rhubarb Buckle with Ginger Crumb, Pear Cobbler with Shingled Hazelnut Biscuit, and Mimi's German Apple Cake are some of my favorites. Many of the recipes bring new twists to old-fashioned techniques, adding new life to slumps, grunts, pandowdies, and buckles. This is by far my favorite new cookbook of the past few years. It's laid out seasonally, and the authors are both from Oregon, so it's a perfect Northwest Farmers' Market companion. It even has a baking dish equivalents chart, which gives volumes and dimensions for different sized pans.
A year after buying it, I'm still smitten with 660 Curries by Raghavan Iyer. It's not a vegetarian or seasonal book, and the recipes can seem daunting at first, but I learned a great deal about processes and flavors from this book. There are many recipes for making your own garam masalas, and Shoreline Central Market carries nearly every spice you'd need in bulk. If you want to add complex flavors to just about anything, make a few of these masalas and keep them around to spice up just about anything. Also, after making 3 or 4 recipes from this book, I had a better understanding of what "curry" can mean, and I felt comfortable throwing together a dish sans recipe.
Chronicle Books has a "Quick and Easy" series, and I've heard rave reviews from friends for the Indian and Korean books. They also have titles for Thai and Chinese. However, they are not vegetarian or seasonal.
Hope this helps!