This has already been done, in a way. A guy took a bunch of newspapers, and tallied up the numbers of times each letter occurred. He did this over a long period of time, and came up with a ranking chart. This list can be used to crack monoalphabetic ciphers, using frequency analysis (the most common code letter would translate to be most common real letter, and so forth). This is how you crack the Cryptoquotes in the newspaper.

In terms of frequency, here are the percentages (out of _The Code Book_, by Simon Singh, page 19):

• a: 8.2%
  
• b: 1.5%
  
• c: 2.8%
  
• d: 4.3%
  
• e: 12.7%
  
• f: 2.2%
  
• g: 2.0%
  
• h: 6.1%
  
• i: 7.0%
  
• j: 0.2%
  
• k: 0.8%
  
• l: 4.0%
  
• m: 2.4%
  
• n: 6.7%
  
• o: 7.5%
  
• p: 1.9%
  
• q: 0.1%
  
• r: 6.0%
  
• s: 6.3%
  
• t: 9.1%
  
• u: 2.8%
  
• v: 1.0%
  
• w: 2.4%
  
• x: 0.2%
  
• y: 2.0%
  
• z: 0.1%