Essential resources for studying environmental issues for the UPSC include reading materials like NCERT textbooks covering biodiversity, ecosystem services, and environmental policies. Books such as Shankar IAS Environment offer comprehensive coverage of environmental science and current issues. Additionally, governmental reports, like Forest Report and State of Environment Report, provide updated data and policy insights. Keeping abreast of current environmental challenges through news sources and journals is vital for a holistic understanding of this subject in the examination.