Why is Shemitah in Sefer Vayikra (Torat Kohanim)? Rav Nissin Alpert, R' Aharon Levine (the Reisha Rav), the Meshech Chochmah.
The real theme of Sefer Vayikra is Kedusha, Parshat Behar focuses on Kedushat Ha'aretz. The parallel between Shemitah-Yovel and Shabbat-Yom Tov.
Why does Yovel require all of Bnei Yisrael to be living in Eretz Yisrael while Shemitta does not?
The Zichron Meir on Tzedakah.
The Lubavitcher Rebbe zt"l on taking interest.