I forgot to mention Morning Sun books, their Color Guide To Freight & Passenger Equipment is very useful, but mainly includes pictures from the 60's onwards, although many of the prototypes are earlier. They do other books on the N&W steam era, which if as good as their color guides should be very good. They are not cheap, and tend to go up in price if you're not careful, but good secondhand ones can be found in the UK (I just picked up both Chessie & N&W guides for the price of either new ).
Mick.