The following sites have been Ironman approved:

Jon Roma: Good site for dates about closings and razings
http://www.jonroma.net/towers/

Dave Haynes: Another good site for towers,lots of track diagrams with signals.
http://members.trainorders.com/nkp759/towerindex.html

Dick Carpenter has written four volumes published by John Hopkins University press called:
"A railroad atlas of the United States in 1946"
Mr. Carpenter has Volume 5,Iowa and Minnesota,out now.
They show all the interlockings in service in that year.

Rich White: A good site that focuses on the nuts and bolts of interlockings and signaling.
http://www.rrsignal.com  


Mike Condren has a site that concerns with railroading in the Memphis Tennessee area. There is a section on towers that will interest you.
condrenrails.com

The TrackSide Photographer is another "Ironman" approved site. The focus is not on trains per se but other parts of railroading;bridges,structures,track,equipment.
Great site..... thetracksidephotographer.

Harrisburg PA. NRHS has done a great job of restoring Harrisburg tower. Here is the link to their site.

http://www.allaboardwaldwick.org/