The study material provided by IGNOU is elaborate and more than enough. You don't really need books except for these 3 subjects:

MEC 001 Microeconomic Analysis: For the subject, you might need some extra notes. The best way is to google the topics. You can make notes on the numerical part especially. Usually, the numerical questions are the same as the previous years. Still, it is recommended that you don't skip the study material.

MEC 003 Quantitative Methods in Economics: For this subject, you can make the best use of YouTube videos. Also practising from the previous year's papers will prove to be very helpful.

MEC 105 Indian Economic Policy: The study material provided for this particular subject needs to be updated with the latest data and figures. So it's highly recommended that you prepare topic-wise notes after thorough study.

Ultimately focus on the syllabus whichever source you are referring to.