IGNOU Public Administration material is very simple and lucid, covering the majority of the UPSC Public Administration syllabus. Before moving on to books by Mohit Bhattacharya and Administrative Thinkers like Prasad & Prasad, IGNOU material is a good starting point. The same is true for the other subjects. So, if you are reading the subject for the first time, you can go through them to get a basic understanding of it.
It is recommended to read blogs on the subject, consult standard textbooks used for the specific course at university, and, most importantly, form a study group. For serious discussion on topics, this group should stay connected on a messaging platform like Whatsapp and the social media of your choice. If you do this religiously, your IGNOU and UPSC journeys will be smooth and enjoyable!
Disciplined UPSC candidates refer to the study material of this Open University's PG program honestly.