Study Material
Lexicon is the starting point for most aspirants. After that, the next logical steps can include following things -
1. Previous year’s papers - It's the most authentic source in order to understand the scope of the syllabus and type of the questions asked.
2. Preparing from study material provided by the institute.
Class notes vary depending upon the background of the teacher. Teachers in ethics come from either of three backgrounds - philosophy teachers, psychology teachers and public administration teachers. They all bring their own perspective which is useful and limiting at the same time. Idea is to incorporate each ones expertise into the content.
3. As far as standard published books are concerned, many people go for a book by Subba Rao as it gives somewhat balanced approach between theory and practice. There are generally three kinds of authors in this field - some successful candidates have written books based on their experience, some university professors and academics have authored books, and finally there are some practitioners like serving bureaucrats. There are also compilation of articles, essays and speeches by combination of above three. Each type of book has its pros and cons.
In ethics, difficulty is that notes and books become outdated quickly as the question paper format keeps evolving every year. What's good today may not be for tomorrow. So, choose the book wisely.
4. Internet - There are very many sources here and some very tempting. There is a course by Harvard Prof. Michael Sandel called Justice. It's a fantastic series of video lectures exposing an uninitiated to the world of ethics. There are various topical videos on YouTube. Various western governments and institutions like OECD or UN have there own documents, kits and reports (like Nolan committee, Cadbury committee of UK)
Indian government has published some reports like Second ARC. DoPT has published a good manual on ethics for civil servants with Indian perspective. Home Ministry, DARPG also have their own manuals.
So, go through previous year’s ethics papers, notice the changing trend and choose your study material accordingly.