The book Codes, Ciphers and Secret Writing by George Beal gives the following information about frequency tables used for cracking simple substitution ciphers:
Letters in order of frequency (highest to lowest):
E T A O N R I S H D L F C M U G Y P W B V K X J Q Z
The letters can be grouped further by their frequencies:
Very Common: E
Common: T
Next most common: A O N R I S
Less Common: H
Less Common Still: D L F C M U
Rare: V K X J Q Z
Other frequency tables have been worked out for pairs of letters:
TH HE AN RE ER IN ON AT ND ST ES EN OF TE ED OR TI HI AS TO AR OU IS IT LE NT RI SE HA AL DE EA NE RO OM IO WE VE TA TR CO ME NG MA CE RA IC NS UT US BE UN CH WA SI LA AD LI RT CA NC SO NC SO LL UR EL RS EM AC IM PR TT OT WI EC
The most common words in English are:
THE OF AND TO IN A IS THAT FOR IT BY ARE BE WAS AS HE WITH HIS
Anyways all of these tables can be used to decipher substitution ciphers. If a certain character appears a lot you might start off by assuming it's an 'e' and work forwards from there using common pairs and words. This is called frequency analysis. Doesn't always work but it's a good place to start right? Of course when used in conjunction with a transposition cipher, that could mess you up for a long time.