Here are some seforim that I find very useful/inspiring for this time period.
שער הגמול - by the Ramban. This sefer spells out many of the Yesodos Haemuna that relate to this time period. It explains what we are judged for on Rosh Hashana, צדיק ורע לא, Gehinom and Gan Eden, etc. It is both a tremendous source of mussar and knowledge for this period.
קונטרוס עבודת התפילה - This explains every line of the shemoneh esrei giving you all of the sources. I find it helps my כוונא tremendously. I like the sefer in Hebrew but there is an English version as well.
אסופי מערכות ימים נוראים - This is a set of essays from R' Goldvicht (former R' Yeshiva of Kerem B'Yavneh and talmid of the Chazon Ish and the Brisker Rav). R' Goldvicht is a master in aggadda and he has some beautiful ideas about the ימים נוראים.