This is the correct forum
Generally you want the heaviest load to be centered over an axle -- So I would think that the back seat would make more sense. But this is just a guess on my part.
Mark
Keep the weight 'in' the car
Definitely over or forward of the rear wheels, and as low as possible in the vehicle. You will find the car much more stable on the road. If at all possible, I would pack as many as you can between the front and the back seats. If you have to put any on the back seat, use the seat belt to hold them. Even hard braking could have them joining you in the front seat.
I once drove a car which was packed by an amatuer, with many heavy items behing the rear axle. It was not long before it started fish-tailing. After repacking everything and putting the heaviest items over and forward of the rear wheels, it behaved itself for the rest of the journey.
I have never hidden normal luggage in a car, and never had a problem. Park in safe, well lit places and don't leave valuables in the car.
Lifey
Much more enjoyble ride if
Much more enjoyable ride if the weight is centered over the rear axles. I drove a new pick-up for the USFS and we welded some steel weights in the bed over the axles to keep the darn thing from bouncing too much! So loading the passenger area with the weight is always going to be good thing. And like glc mentioned -- 300 pounds of books is nothing!
Mark