Well, you certainly can't go wrong with any of Julia Child's books - particularly her first 2-volume set on traditional French cooking. They can be found very reasonably in 2nd-hand bookshops - in the flesh & on-line.
After that, if you can find - again in 2nd-hand shops - the French volumes of the two Time-Life series on international cooking, once again you'll be both lucky & well-taught. In fact, even after many, many years of making it, I still use the Time-Life French cookbook version as a base for making my own adaptation for an annual traditional New Year's Day French Cassoulet.
Third, France The Beautiful Cookbook is another standby of mine for authentic, if newer, French recipes.