In my latest edition of the Institute of Railway Signal Engineers magazine, there's an article on a book published by Steve McEvoy about a signal tower at South Norwalk, CT. This tower is no longer an active tower but does still sit alongside the busy four-track mainline between Boston and NYC.
The book itself is described as easily accessible; not too technical and "fills a huge void in books on N.American signalling, and does a wonderful job". The book is entitled "The Classic Railway Signal Tower", ISBN 978-1-59872-858-3. http://www.westctnrhs.org/giftshop.htm
I have no affiliation with either the book or the tower, but those that are interested in signalling and who are in the area might like to take a look and report back. It's certainly something I'd do on a side trip next time I'm stateside.