Then said I, Lo, I come: in the volume of the book it is written of me; rather, then said I, Lo, I come with the roll of the book written concerning me. "Then" means "as soon as my ears were opened."
"Lo, I come," marks ready and prompt obedience (see Numbers 22:38; 2 Samuel 19:20). The psalmist represents himself as brining with him "the roll of the book," i.e. the book of the Law in its ordinary form of a parchment roll, to show what it is that he is prepared to obey.
This book, he says, is written "concerning him," since it contains precepts concerning a king's duties (Deuteronomy 17:14-20).