In preparation for the upcoming holiday, here are some relevant free books available on HebrewBooks.org:
- Gevuras Yitzchak on Elul, Rosh Hashanah and Yom Kippur by R. Yitzchak Sorotzkin (Lakewood, 2007)
- Nitei Gavriel on Hilkhos Rosh Hashanah by R. Gavriel Zinner (Jerusalem, 2001)
- Derashah Le-Rosh Hashanah by Ra’avad of Posquieres
- Otzar Ha-Mo’adim by R. David Dov Meisels (NY, 2001)
- Gam Ani Od’kha on Rosh Hashanah by R. Gamliel Rabinowitz (Bnei Brak, 2009)
- Yalkut Le-Mo’adim on Rosh Hashanah and Yom Kippur by R. Chaim Becker (Jerusalem, 1967)
- Sefer Ha-Mo’adim on Rosh Hashanah and Yom Kippur by Dr. Y.L. Baruch (Tel Aviv, 1946)
http://www.hebrewbooks.org/43915
Ateret Rosh, by the 2nd Lubavitcher Rebbe, on RH/YK.