Best SAT subject test Prep books for Physics, French, and U.S. history?

I’ve heard that Barron’s is the most difficult…is it unecessarily difficult or does the extra difficulty help you do better on the exam? I’m hoping to get 700+ in U.S. and Physics and 600+ in French.

What are your experiences with each book and which do you think is the best?

Also, how hard is the French test (if you’ve taken it)?–I’m in IB french (equivalent to AP French or 5th year in French), but I’m only mediocre at French.