Cookbook for an accomplished baker?

Adding to the list of wonderful books already suggested:

Baking by Flavor -- Lisa Yockelson. The book is laid out, as the title suggests, by flavors -- banana, chocolate, cinnamon......vanilla.
Heavy emphasis on using vanilla and lemon sugars, double-strength vanilla, chopped candy bars, and other flavor-intensifying ingredients.

I agree with most of these. I'm partial to Maida Heatter's cookbooks-- I learned to bake from them. I also love Kim Boyce's Good to the Grain and Alice Medrich's books. If your friend doesn't already have it, consider Shirley Corriher's Bakewise. Not so much for the recipes, but for the science behind them. To get to the next level, I think it's good to have an understanding of the ingredients and why they're used as well as what happens when changes are made. Shirley has helped me be a more creative baker.

Rose Beranbaum Heavenly Cakes

Another of my favorite dessert books is Nick Malgieri's "Great Italian Desserts." He traveled all around Italy for this book, "stealing" recipes from Italian grandmas and pastry chefs alike. If she likes to cook Italian, I'll bet she'll really enjoy these authentic desserts.

I agree with the recommendations for the Bouchon Bakery Cookbook and anything by David Lebovitz. One I would add that I haven't seen mentioned yet is the first Tartine book.

I consider myself to be a pretty competant baker and my go to book is my old textbook. The Professional Bakers' Manual by Rudolph and Sohm, because it gets into the science of baking. I also highly recommend the Bouchon Baking book. My third suggestion is Baking by James Peterson, which has lovely step by step photo spreads which are incredibly helpful to a visual learner.

I also love to bake and that is the largest section of my (huge) cookbook collection. If your cousin already owns basic baking cookbooks, I think she would probably appreciate books that have more unusual and interesting recipes. Other factors are whether she needs photos and if she prefers more modern books.

I agree with the Shuna on Alice Medrich's books (especially if your cousin likes chocolate), Chez Panisse Desserts and Jane Grigson. Others I enjoy using are:

David Lebovitz's - Ready for Dessert, Ripe for Dessert and Room for Dessert (this one is hard to find)
Dorie Greenspan - Baking: From My Home to Yours (and she has a new one Baking Chez Moi coming in the fall)
Dede Wilson - Bake It to the Limit and Unforgettable Desserts
Nancy Baggett - all her cookie books are great
Annie Bell (a British author) - Gorgeous Cakes, Gorgeous Desserts and The Baking Bible

I second most of these suggestions, and to it I will add:
Dan Lepard: The Handmade Loaf
Alice Medrich - any cookbook but especially: Pure Dessert
Jane Grigson: The Fruit Book [this is my "bible" - no other book teaches more about fruit through cookery
Linsdsey Shere: Chez Panisse Desserts
CIA Hyde Park: The Baking & Pastry book by
Francisco Migoya: The Modern Cafe - this book is only for the uber serious, but there are some gorgeous ideas, and benchmark examples
Nancy Silverton: Pastries From La Brea Bakery -
I agree about the Cake Bible, but that book is mostly about cakes, and there's a whole world of pastries out there for a great baker.

One that's a little different that might offer your cousin some new techniques is Beatrice Ojakangas's Scandinavian baking (that might not be the exact title). I am not a super-baker but I love it - and a friend who IS a super-baker also loves it.

Kim Boyce's Good to the Grain. Karen DeMasco's The Craft of Baking. Use 'em all the time.

I agree with those who suggested Rose's Cake Bible and Baking with Julia. I would also suggest Rose's Pastry Bible, anything by Maida Heatter, and the Miette book. Gift her that last one and ask for a Charlotte in return :)

The Bouchon Bakery Cookbook has a ton of great recipes with a few challenging techniques. Definitely helps refine skills!

I enjoy everything by Alice Medrich. She has a handful of baking cookbooks, so you can get anything ranging from "Chewy Gooey Crispy Crunchy Melt-In-Your-Mouth Cookies" to the ultra-specific upcoming "Flavor Flours."

A lot of her stuff works for beginners, but also serves as a next-level cookbook.

Any book by Rose Levy Beranbaum will be easy to follow, but will have recipes that will be challenging. And SO worth it! But when I first read your question, my thoughts turned immediately to "Baking with Julia." It's kind of a "vintage" book, but it's still available on amazon. It's from a TV series where Julia Child baked with a number of well-known bakers, and the recipes are superb.

If she has a sense of humor, check out Surprise-Inside Cakes by Amanda Rettke.

Let me add that the Cake Bible has several recipes that I think are best made by an accomplished baker. So it might be a good fit.

Cake Bible by Rose Beranbaum.

I love Joanne Chang's Flour cookbook