This is the second book of the Mishneh Torah of the Rambam, Rabbi Moshe ben Maimon. Laws of the Sefer Torah, Tefillin, and Mezuzah.
This chapter explains whether the crown of each letter on a Tefillin, Mezuzah, or Sefer Torah prevents the fulfillment of the Mitzvah or not; ten fundamental laws concerning Tefillin, dictated to Moses on Mount Sinai; the difference between these three matters regarding the skins on which they are written (none of the three are written on the skins of impure animals or fish); who is appropriate to write them and whether they were written without copying from another text; all the laws of how they are written, how the lines are marked, and how the letters should not be touching each other; the names of God and all His works.