As you know that IGNOU (Indira Gandhi National Open University) the largest university in the world with 3500000 students was founded to encourage, coordinate and set standards for education in India.
Because of IGNOU notes content, quality, authenticity, affordability and outstanding depth of knowledge it is very popular in IAS aspirants community. IGNOU notes has been developed for the likeminded students to help them and achieve their goals. It's like on panacea for IAS candidates. Almost every year UPSC asked questions from this unique notes. It can be extremely useful in IAS optional subjects like (History, Sociology and etc.) if you will read it strategically it will definitely enhance you knowledge.