The following books may be of great spiritual depth, and possible guides through this season.
Days of Awe, Agnon, the Israeli Nobel Laureate. It is a stunning collection of wisdom on the themes of Repentance and Turning culled from every period of Jewish learning.
Entering the High Holydays, Rabbi Dr. Reuven Hammer (Jewish Publication Society). A guide to the services and themes of the High Holydays by the past president of the (Conservative) Rabbinical Assembly and a professor at the Schecter Institute in Jerusalem.
60 Days, Rabbi Simon Jacobson. A spiritual guide to each of the 60 days between Shavuot and Sukkot. Provides a great gift of combining insight from the kabbalistic and Hassidic traditions with an understanding of the spiritual quest of modernity
This Is Real and You Are Completely Unprepared, Rabbi Alan Lew, published by Time Warner. An eclectic but profound approach to the themes of repentance, turning and prayer.