To date, my favorite bathroom book that I've bought is Cars Detroit Never Built, by Edward Janicki. Some are a laugh but others are really forward-thinking.
Made to "combine American design tastes with European-inspired body styling", the Chrysler K-310, with a face only a mother could love:
The 1954 Firebird I, god only knows if they thought anybody would buy one. On the plus side, it had a lighting system that turns on automatically when it's dark and keyless entry.
Another face only a mother could love, the 1955 Chevy Biscayne:
My vote for the wave of the future, modular rear ends. More literally convertible than your Boxster or Miata, the 1979 Ford Fiesta Fantasy:
Ford's "urban commuter car of the future", the 1981 Cockpit, a three-wheeler with tandem seating:
Finally, the 1992 GM Ultralite, powered by a 1.5L, 3 cylinder two stroke. With a 420-lb carbon fiber monocoque and a drag coefficient of .192, it got 80 mpg and went from 0-60 mph in 8 seconds. It also had an active suspension to maintain the best ride height for aerodynamics.
The funny thing is that the concept cars tell you more about the time they were made than what the car of the future will be like. When the manufacturers are under pressure to improve fuel mileage, all the concept cars are suitcases on wheels, and one almost gets the feeling they intentionally make them unattractive. Then fuel is no object and they're under pressure to get people "excited in the brand" and boom, you've got the return of the Camaro and Challenger.