Charles Ellicott Commentary Psalms 40:7

"Then said I, Lo, I am come; In the roll of the book it is written of me:" — Psalms 40:7 (ASV)

Then said I. —This rendering, which follows the Septuagint and Vulgate, and is adopted in the Epistle to the Hebrews, must be abandoned. The Hebrew means, Behold! I come, bringing the book written for me, which no doubt refers to the Law, which in the person of the poet, Israel here produces as warrant for its conduct. Some see a particular allusion to the discovery of the Book of Deuteronomy in Josiah’s reign. But if the conjecture of Grätz is accepted (see preceding Note), the reference will be rather to the Levitical regulation of sacrifice. “If you require burnt-offering and sin-offering, then I say, Behold! I bring the book in which all is prescribed for me,” i.e., I have duly performed all the rites ordained in the book.

The rendering “written on me,” i.e., “on my heart and mind,” might suit the contents of the book, but not the roll itself.