Possible Clues:

• Source of quote
• Horton's creator
• Creator of Sylvester McMonkey McBean
• Creator of Aunt Annie and Benjamin B. Bickelbaum
• Writer who used his actual middle name as a pen name
• Source of the line 'There is no one alive who is you-er than you!'
• His birthday is Read Across America Day
• He created Whoville
• Who wrote 'Unless someone like you cares a whole awful lot, nothing is going to get better. It's not'
• Sam-I-am's creator