My fiancee and I love to cook. We love cookbooks, but often find that they are useless in today's internet world where all the recipe sites exist. Mastering the Art of French Cooking is a classic, which is not just a list of recipes (as I have found many other "cookbooks") but something more. I want to buy it for her--but the problem is that we keep a kosher kitchen, i.e. we don't mix meat and milk. Child is notorious for her ubiquitous use of butter and cream. Is this book useful in a kosher kitchen, or is it pointless since all of the recipes will call for mixing milk & meat? I have a sense that for many of the recipes, you can simply cut out the butter, but isn't the point of having a recipe to follow it?
I think you should take it out from your public library and take a good in depth look at it. This way you don't have to buy it and you can look at all the recipes. If you see enough recipes in there that you don't have to change, or just have to change minimally, then you can justify buying it. If your library doesn't carry it you could either get an interlibrary loan from another library somewhere else or you can always request if from them. Most libraries are really good about buying books (cds, dvds, etc).
Unless you're the type that has to follow a recipe to the letter, then buy the book. We keep a kosher kitchen, and we get ideas from "trayf" cookbooks all the time.
One of the things I like about Mastering the Art of French Cooking is that the recipes go into great detail about how to do things. For example, Julia Child's recipe for roasted chicken does use butter (for which, of course, you can substitute pareve margarine for a slight change in taste), but it is also 3 pages long and includes suggestions for side dishes, wine recommendations, and instructions for how to make a sauce from the pan juices. Just reading the recipe taught me a lot about the whole philosophy about eating in the French style -- it's not a recipe just for chicken, but it's a whole meal thought of as one unit.
It's also fascinating because the vegetable recipes are for the most part cooked for much longer than currently suggested and cooked with lots of butter and cream.
I'll tell you that I don't use the cookbook as often as some of my others. (And I also keep a kosher kitchen.) But I love reading it to get ideas for meals and for preparation. And as Janelle pointed out, you can substitute for a lot of the dishes. In fact, margarine and fake cream (soy or whatever) will serve in the majority of dishes that don't contain pork or shellfish. And the majority of vegetable and fish dishes will do just fine as is for a dairy meal.
It might be usable, but probably not worth purchasing. My recommendation would be to look at a book index.
In addition to all the butter substitution, which will not work with all recipes, there are also shellfish recipes, such as lobster thermidor which will be useless to you. The dessert recipes are probably all usable, but I'm not sure if its worth it. I guess it depends on your fiancée, I would be frustrated by a bunch of recipes I could never use.
I think it would still be very useful. I'm vegetarian, so you can imagine what a high percentage of Mastering the Art is recipes that I can't directly use. But what I've learned about technique from that book is priceless.
I also keep a kosher kitchen and love my copies of "The Joy of Cooking" and "Mastering the Art of French Cooking." There are substitutions for many of the ingredients (Krab instead of Crab, soy meat replacement), the recipes and the techniques are wonderful.
I'm not a Jew and I don't practice kosher. I have a rough knowledge on kosher as "don't cook/eat meat and milk (dairy products) together" or "don't eat pork". I also know that Kosher is a much broader concept, not limited with those two bans. French cuisine, as a whole, is obviously not kosher in any means, even by the most limited definitions of "Kosher" and "French Cuisine". So, Mastering the Art of French Cooking is not a book written for kosher practice. Modifications is up to your beliefs, up to the level you practice kosher, I suppose.