We seem to have two types of answers to this query:

1) What books will help a rail preservation site operate better;
2) what we who call ourselves rail preservationists should be reading, be it to further the cause of rail history preservation or just to make restroom dwell time more enjoyable, for all we know.

To the latter, I would simply say: Whatever furthers the education of YOUR particular sub-set of rail history interest. Both the specific (the book covering your hometown line), the general (the book on all the railroads in your state, or all interurbans, or whatever), and the very general.

The books that get the most use and abuse at this desk, for example, include Drury's Historical Guide to North American Railroads, the Marre/Pinkepank Diesel Spotter's Guide/Diesel Locomotives series, the American Shortline Railway Guide series, the SPV rail atlas series, and nowadays the Middleton/Smerk/Diehl Encyclopedia of North American Railroads. In addition, various books on rail stations, bridges, the White books on locos and cars, the Hilton book on narrow gauge, and various books on British rails, and the Taber Railroads of Pennsylvania Encyclopedia and Atlas.

I was utterly blessed to blunder into a well-hidden copy of Lines of Country: An Atlas of Railway and Waterway History in Canada by Christopher Andreae some years back at a bookstore-chain bankruptcy clearance sale for only $10 when its list price was $95, and it has paid for itself more than any other book in terms of information per dollar spent; you'll not find it for under $180 these days, I wouldn't think, and more likely $300+.