Below are links to websites that I have found useful in doing my research.
For the general history of postcards in the U.S.: Vintage Postcard History.
For estimating the date of manufacture based on the style of a postcard's stamp box: Playle.com.
For estimating the date of postcards printed by the Wayne Paper Box & Prtg. Corp., Ft. Wayne, Indiana: dejean.com.
DATING OF EXISTING STRUCTURES
If you know the exact address of the house or other building, you can search on it at the county assessor website for Lake County (when your search pulls up the correct page, you will need to click on "Improvement Information" to get building dates) or Porter County.
If you do not know the exact address, you will have to find your structure via satellite view on the GIS sites for Lake County or Porter County.
These records cover only structures that are still standing.
VINTAGE LOCAL HISTORY BOOKS ON-LINE
Ball, T.H. Encyclopedia of Genealogy and Biography of Lake County, Indiana (1904).
Chicago Historical Society. Chicago Photographers 1847 Through 1900 as Listed in Chicago City Directories (1958).
Goodspeed, Weston, and Charles Blanchard. Counties of Porter and Lake Indiana. Historical and Biographical (1882).
Howat, William F. A Standard History of Lake County, Indiana, and the Calumet Region (1915): Volume I and Volume II.
Goodspeed Brothers. Pictorial and Biographical Record of La Porte, Porter, Lake and Starke Counties, Indiana (1894).
Woodcock, E.E. and Clyde R. King. Souvenir Album of Lake County, Indiana (1906).
Woods, Sam B. "The Passing of the Pioneers" (1929).
Writers' Program of the Work Projects Administration. The Calumet Region Historical Guide (1939).