Elul and Yom Kippur are flying by fast. I am looking for something uplifting.
Books on teshuva are usually quite depressing; too much talk about gehinom. Not that it's not good, just that it doesn't work so well for me.
Gates of Repentance (Sha'arei Teshuvah) by Rabbeinu Yonah of Gerona.
I would recommend Igeres Hatshuva by Rabbi Shnuer Zalman of Liadi. An excellent translation and commentary is available online.
Reb Meir Stern teaches Rambam Hilchos Teshuva like I have written elsewhere. I would recommend you listen to a shiur on the Rambam from YUTorah etc.