If I hear of a cookbook I want to buy, Amazon is the first place I look because so often their prices are best. But recently I have had trouble finding some of the titles I want, mostly because I am looking for titles that are out of print or the very specific high-end professional ones that Amazon doesn't carry. For example -
Laduree: The Sweet Recipes
L'Art Des Petit Fours Sucres et Sales - There is an edition of this that has recipes in both English and French, but it is hard to find
Sweet Cuisine by Bau
You also asked how we use cookbooks - For me, I have two categories: Home and professional. I don't buy many cookbooks for home cooking any more because I have so many already, but I do use them a lot to plan meals for my days off. These are often simple, comfort food-type recipes.
For work, I am a pastry chef and will often flip through my professional cookbooks when planning menu changes. I want my desserts to be my creations, but the cookbooks are great for giving me plating ideas, flavor combinations or sometimes I just need a technique for something I want to achieve. For example - Pierre Herme has a recipe for lemon cream in his dessert cookbook that Dorie Greenspan also includes in one of her books and it is by far the best I have ever made. I have since made it in lime and even sour cherry and the texture is phenomenal. When you find one great base recipe like that, it is worth the price of the book because you can use it in so many applications. Other books, like Frozen Desserts by Migoya is great for understanding all of the basic ratios for semifreddo, souffle glace and such. I reference it a lot when I am troubleshooting problems with a new ice cream flavor or wanting to tweak the texture on something.