I've been looking since quite some time for books on gourmet vegan kitchen, that require a lot of competence, and I even asked here on the board for advice. That time I got recommended "Crossroads" by Tal Ronnen, which is a great book in my opinion and I'm very thankful for that answer! Besides that, I'm having a very hard time searching for similar books, so I was wondering if it could be useful to go for books like "The french laundry" by Thomas Keller, which is not aiming for a pure vegan cuisine (allthough they may contain some vegan recipes, couldn't figure that out). Still I could imagine, that books like these contain a lot of knowledge that can be used for vegan kitchen aswell. Is that a correct assumption, or doesn't that make sense at all? 
The usefulness of the cookbook is going to depend a lot on what you, personally, need from it. Take The French Laundry Cook Book; it's got a ton of information in it, but it's mostly organized in separate, specific recipes.  So, for example, the book includes this piece of generally useful information:  Chestnut is a luxurious wintertime filling, and a great flavor combination with the Italian Fontina cheese, celery root, and truffle oil  And on the same page there's a note about how to peel chestnuts, which is also generally useful.  But would you know to look for this note within the recipe for chestnut agnolotti with Fontina and celery root purée? There's also the problem of knowing what information is generally useful, and what's only useful for a recipe very similar to this one.  For example, this same chestnut agnolotti includes this note:  Pass the sauce through a chinois into a large skillet.  You can use the back of a small ladle to help the liquids pass through the strainer, but do not force any solids through.  The instruction to use a ladle, but gently, is actually a useful technique, but because the reasoning is not explained (i.e. it doesn't say why you don't want smashed bits of potato in the finished sauce), this is probably only helpful to a certain kind of reader. If you're following instructions somewhat blindly, you won't know what you can apply elsewhere.  But if you come to this recipe already having a decent idea of what it's trying to achieve, a brief note of instruction might be just enough to teach you when to use this technique. For contrast, I'll grab another French cookbook off the shelf: Julia Child's Mastering the Art of French Cooking.  This book is far more useful to the novice cook because there's a far higher ratio of explanatory notes to specific recipes. A good example is the chapter on vegetables (chapter 8, if you're following along at home).  It begins with an explanation of basic vegetable cooking techniques, and then provides recipes for specific vegetables, many of which use the general techniques just described. Look at the beginning of the recipe for braised artichokes:  Most of the many recipes for braised artichoke follow the general lines of this one.  You may, if you wish, add to the casserole a cup of diced tomato...Another suggestion with different vegetables follows this recipe.  Braised artichokes go well with roast or braised meats, or they can constitute a first course.  There's a lot of general information in there: most recipes work like this one; you could add these extra ingredients, or this other set of ingredients; this kind of dish is good with these other foods. For the experienced cook, this is probably information they don't need.  It's too general.  But for someone who's not confident about cooking artichokes, this is probably significantly more useful than a very precise description of how to make one artichoke dish. Both kinds of cookbooks have a lot to teach readers.  Which kind is more useful is a question about you, as much as it is about the book. 