Write this song [the Torah] for yourselves....
One should either write or buy a sefer Torah and
other sefarim from which to learn.
The Rosh explains that the mitzvah nowadays includes
buying those sefarim which one will study from
on a regular basis.
The Midrash says that a person should record his
learning and that will be considered as an offering of
atonement to Hashem (Midrash Tehillim 48:2).
This mitzvah is the last of the 613 recorded in the
Torah. The first is the mitzvah to be fruitful and multiply,
in Parashas Bereishis. These two mitzvos encapsulate
the purpose of a Jew’s life: “Produce a nation that
will serve Hashem” (Rambam, Moreh Nevuchim 3:51).