Well, it never was and nor would it be ever.
E. Bharucha Book on Environmental Studies by UGC is the only needed holy book for this alongside the last year papers.
Frankly speaking, I’ve seen people bunking lectures and getting away with really good scores even with 2–3 days of focused and oriented study.
You must have a proper look on the paper-setting in past years and then work accordingly. It has the second Highest credits in the CBCS SYSTEM and believe me, if you are good at memorizing, presentation and note-making then you can score a perfect 10. These papers are checked at the college level and not in the University.
Note making and memorizing facts (e.g food webs, important environmental movements, trending topics like Global Warming, Ozone Layer Depletion, Rainwater Harvesting) would help you a long way in scoring above average marks.
Personal Advice – Treat your Core Papers seriously and focus less on this paper because you can hit amazing scores even with less practice (smart but not hard work)