Nusach Ashkenaz has these separate laining modes:
- Regular weekly Torah reading (Shabbat, weekday, Yom Tov, etc.)
- Special Torah reading mode for Yamim Nora'im (High Holidays)
- 3 Megillot - Kohellet, Shir Hashirim, Ruth
Each tune is supposed to create a "mood" / nuance appropriate for that holiday. E.g. - Eicha and Esther are in major (mainly), though Esther is for Purim which is a happy occasion and Eicha is on Tish'a B'Av which is a sad occasion. Both tropes are similar to each other, yet different from the rest. There's probably a reason or "setting" for the tune / note scales chosen for each holiday.
Is there a resource that explains the nature of the different trope moods?