We can highly recommend the following resources:
The Deracheha website has a number of articles on the topic of tzniut. See here for a conceptual introduction, and here for a more specific discussion of clothing. Choose the “In Depth” tabs for a detailed source-based discussion.
Rav Getsel Ellinson’s book, The Modest Way (in Hebrew, Hatzene’a Lechet) is an accessible compendium of traditional sources, from the gemara to more modern responsa, and includes discussions of dress.
Our site’s founding halachic supervisor, Rav Yehuda H Henkin z”l, wrote an important and comprehensive article on the topic. It can be found online in Tradition, and is the title essay in his book Understanding Tzniut.
Behatzlacha!
This response was updated on 3 May, 2021.