Possible Clues:

• American author who published his first short story at age 30
• Evangelist, a twin possibly and a great writer
• Tom Sawyer's creator
• 'I respect a man who knows how to spell a word more than one way'
• Creator of Tom Sawyer and Huckleberry Finn
• Many would say he was worth one apostle and a couple more
• Becky Thatcher's creator
• S.Clemens
• Mk II version Tom had him writing about?
• A wit badly described by brand new US novelist