In November 1938, Bob Feller turned 18. That previous summer he pitched 62 innings for the Indians and struck out 76 batters. That ratio may not seem that unusual these days, but in that era, it was incredible. He struck out 27.2% of the batters he faced, 3.33 times the league average. Most of what I do below is graphical. For more statistical details click here.
The graph below shows the major league strikeout leaders from 1901-2007. Feller led from 1938-1941 with at least 240 in each season. In the previous 23 seasons, 240 had only been topped once. The leaders fell off until Feller had 348 in 1946, the highest since Rube Waddell had 349 in 1904. The last time anyone had 300+ was in 1912, by Walter Johnson. You can click on any image to enlarge it.
The next graph shows the major league leaders in strikeouts per 9 IP from 1901-2007 by pitchers who qualified for the ERA lead (not counting the Federalist league, which is true for all graphs here). Feller led from 1938-1940. His 7.77 in 1938 was the highest since Waddell had 7.86 in 1905.
The next graph shows the strikeout per 9 IP leaders for those with 50+ IP from 1901-2007. Feller's spikes are noted and you can see they were unprecedented and it took many years for them to be equalled.
The next graph shows the leaders in strikeouts per 9 IP with 300+ IP from 1901-1980 (the last year anyone pitched 300+ IP). Feller's 1946 season would not be passed until Koufax did it in 1963 with 8.86. Feller had the highest in the century until then. His 1940 and 1941 seasons were very good, too. In 1940 he had 7.34, which had been topped only once in the previous 28 years.