Add my name to the list of those who suggest Mastering the French (McDonald) and studying well annotated/commented games for learning the basics and strategies and ideas.
Let me tell how I reached my level of understanding and kowledge of the French:
I am an 1.e4 player. My scores vs the French left much to be desired. In retrospect, I hardly understood the positions beyond that which was explained in My System or from experience or opening books. I saw Mastering the french at a bookstore, opened it and scanned it. I beheld the explanations, the diagrams with arrows, the plans, everything put at my level. I studied that book inside and out. I started playing over games especially from New in Chess (highly recomended) back in the days and the informator (that was challenging in the beginning). Nothing enlightened me more than that book did. I went back to reviewing those game but with the new background knowledge of its underlying principles and plans, tactics and strategies.
I was doing all of that to learn how to play vs the French better than I did. I gained an understanding I never had or thought I would have. I learned that if White gets and advantage it takes as much skill to keep it as the advantages in the English. I mean they are subltle and positionally too important. The closed nature of the game means that maneuvering and piece placement mean a whole lot very early. I learned the important of development schemes based on play around e6, h4 and h5 squares. I started to see that for white in many cases it is the same scenarion Black has in the KID. I mean, White is trying to hold Black's Q side attack (which has a head start) while he hunts Black's Kingside. In the King's Indian that is what Black is doing. And in the French White is trying to stifle Black's serious counterplay on d4 and the Q side.
White has to learn the tools and the tricks to protect whatever he has gained and be able to continue his own attack since the danger line is thinner than many patzers may think. I mean, you could get in trouble as White easier than you may think.
At this point I am at the level that when confronted with unusual moves or set ups, I know how to navigate and I feel comfortable and, most importantly, I am in greater control than before.
Look at my game vs a FM, which was published by GM McDonald in a rare line which Petrosian used to use. Without that preparation I would have never been able to come up with g4 ! and let alone follow the scheme I did and hold Black's play on the Q-side.

By the way, I got my senior member status with this post. One of my more serious (or somber) post, if not the most serious. ???