I'm in the book, but not on any leaf; I'm in the mouth, but not in lip or teeth; I'm in the atmosphere, but never in the air; I wait on every one, but never on a pair; I am with you wherever you may go; And every thing you do I'm sure to know; Though when you did it I should not be there, Yet when 'twas done, you'd find me in the chair-What am I?