There is a nice write-up in a recent issue of American Philatelist (I think it's November but don't quote me on that - I can't find my copy!) about Tasco information booklets.
These were produced in the late 1930s and early 1940s by the Tatham Stamp Company of Springfield, Massachusetts.
These pocket-size guides helped collectors identify different varieties of sometimes hard to identify issues such as the 1c and 2c Issues of 1890 to 1898 (shown above - click on it to enlarge) , 2c Columbian, Confederate states and the potato stamps of the United States
I had never heard of these before and found them fascinating. After little surfing around on the Web I found a site created by Glenn A. Estus that shows most - if not all - of these highly educational booklets.
To learn and see more of the Tasco booklets, click here.