Step 1: Place each book in its subject area.
Silent Spring is an environmental science book about pesticide harm.
Handbook of the Birds of India and Pakistan is an ornithology reference book.
Small is Beautiful is an economics and technology book.
The Unquiet Woods is a book on social and ecological history.
Step 2: Recall the author tied to each book.
Silent Spring, 1962, is by Rachel Carson.
Handbook of the Birds of India and Pakistan is jointly credited to Salim Ali and S. D. Ripley.
Small is Beautiful, 1973, is by E. F. Schumacher.
The Unquiet Woods, 1989, is by Ramachandra Guha.
Step 3: Map to Group-II and note the leftover name.
P - 1, Q - 4, R - 5, S - 3.
Madhav Gadgil, though a well known Indian ecologist, is not the author of any of these four books, so option 2 is left unused.
Final Answer:
$$ \boxed{\text{P - 1, Q - 4, R - 5, S - 3}} $$