Then - in such case, without necessarily referring to order of time.
Lo, I come - I am prepared to do, etc.
in the volume of the book - roll of the book. Such rolls, resembling maps, are still used in the synagogues.
written of me - or on me, prescribed to me (2Ki 22:13). The first is the sense adopted by Paul. In either case, the Pentateuch, or law of Moses, is meant, and while it contains much respecting Christ directly, as Gen 3:15; Gen 49:10; Deu 18:15, and, indirectly, in the Levitical ritual, there is nowhere any allusion to David.