Why does it say "v'Yoser me'Hemah Bni Hizaher [Asos Seforim Harbe Ein Ketz]"?
Eruvin 21b, partially brought in Rashi: One should be more careful about mid'Rabanan laws than Torah laws ("Yosher Divrei Emes" - 10). Most Torah Mitzvos are Aseh or Lavim [without Misah]. However, anyone who transgresses mid'Rabanan laws is Chayav Misah [b'Yedei Shamayim - "u'Foretz Gader Yishechenu Nachash" (10:8)]!
Rav Sadya Gaon: My son, this is the advantage that you will have from [Divrei Chachamim].
Ibn Ezra: Be careful me'Asos (not to make or buy) many Seforim. Also "Hishamru Lachem Alos ba'Har" (Shemos 19:12, is like me'Alos).
Rashbam, Metzudas David: Be careful about more than what Chachamim warn you, for it is impossible to write Seforim containing all Chachmah [everything that one must be careful about].
Rid: Be very careful not to budge from Divrei Chachamim.
Why does it say "Asos Seforim Harbeh Ein Ketz"?
Rashi, from Eruvin 21b: If you will say, if mid'Rabanan laws are proper, why weren't they written in the Torah? We would not suffice to write it 1 .
Rav Sadya Gaon: Acquire many Seforim without limit.
Rid, Seforno: This refers to "Nitenu me'Ro'eh Echad" (11). All were given to Moshe on Sinai, but were not written, for they were too many to be written.
Now that "Es La'asos la'Shem Heferu Torasecha" (Tehilim 119:126), all oral Torah was written! Etz Yosef (on Ein Yakov 21b) - we could not write fences that must be made, for this varies from time to time. E.g. Rav saw a valley (potential pitfall), and fenced it (Eruvin 6a). Bris Olam (Pekudei DH v'Zehu) - sometimes Chachamim wrote only the Kelalim, but not the Peratim. (E.g. Kelalim like 'Aseh Docheh Lo Sa'aseh' and 'Migo he would be believed to say...', 'one who is suspected about a matter is not believed about it...'; only a few applications of them were written. - PF)
What is the meaning of "v'Lahag Harbe Yegi'as Basar"?
Rav Sadya Gaon: Learn much, until the body is wearied.
Rashi: If one will seek to say more than the heart can understand, this is exertion that one cannot attain. Do not say, since I cannot finish the work, I will not begin. Dunash said, Lahag is an expression of speech; we do not find it elsewhere in Tanach. Ibn Ezra - the Lamed is from the root, like the Lamed in "Limed Da'as" (9). Its grammatical form is like "Lahat ha'Cherev" (Bereishis 3:24).
Rashbam: Lahag Harbe is a repetition (many matters of Chachmah). It is too tedious (and therefore impossible) to write all Chachmah. Lahag is the words of those who write Seforim.
Rid: This refers to "v'Yoser me'Hemah Bni Hizaher." Their words explain, straighten and help understand the Torah. Lahag is learning without understanding; it is exertion of flesh without benefit.
R. Avigdor: Lahag is like La'ag; Hei is interchanged with Ayin. Anyone who mocks Divrei Chachamim is judged [his punishment is to be] in boiling excrement (Eruvin 21b) - Yegi'as Basar.
Seforno: [To try] to write [all that Moshe received] would weary physical power, and a person would not suffice to finish.
Metzudas David: To read many Seforim [warning about everything one must be careful about] is wearisome for people to read and remember. Therefore, one must use his intellect to be careful about what is proper to be careful about it.
Metzudas Tziyon: Lahag is engaging in learning. The Targum is ul'Misak b'Fisgamei Oraisa. A Piyut on Rosh Hashanah (Hashem Melech Hashem Malach..., before Kedushah of Shacharis on the second day) says 'b'Lahag Melahagim.' It is Arabic, like reading. Basar refers to man - "Yavo Chol Basar Lehishtachavos Lefanai Amar Hashem" (Yeshayah 66:23).
