Meleches Koseiv: Minchas Chinuch - The Last Letter Counts
A person who writes two letters
on Shabbos is Chayav Chatas for the Melachah of Koseiv (writing), however
if a person writes the last letter of a Sefer he is Chayav a Chatas
even though he wrote only one letter because he completed the Sefer
with that letter. (Shabbos 104B). Rashi says that this holds
true if the letter that he wrote completed one of the 24 Sefarim of
Tanach.
The Minchas Chinuch explains that the 24 Sefarim of Tancah have
an exact count of the number of letters that it contains therefore if
a persons writes the letter that completes the Sefer he is Chayav. For any other Sefer, even Sifrei Kodesh, do not consist of an exact number
of letters and therefore even if one wrote the last letter of the Sefer,
he did not necessarily complete the Sefer, since it is possible that
letters will be added or subtracted. Therefore in this case he is not Chayav unless
he wrote two letters.