There are several books and resources that can be helpful for answer writing in the Botany optional subject for the UPSC exam. These resources provide comprehensive coverage of the subject and can assist in gaining a deeper understanding of the topics. Here are some recommended books and resources:

1. "Plant Anatomy" by B.P. Pandey: This book covers the anatomy of different plant tissues and organs, providing detailed explanations and illustrations. It is highly recommended for a thorough understanding of plant anatomy.

2. "Plant Physiology" by Taiz and Zeiger: This book is considered the bible for plant physiology. It covers all the essential topics, including plant water relations, mineral nutrition, photosynthesis, respiration, and plant growth and development. The book provides in-depth explanations and is a valuable resource for answer writing.

3. "Genes IX" by Benjamin Lewin: This book is a comprehensive guide to genetics, covering various aspects such as gene structure, regulation, and inheritance patterns. It is highly recommended for those preparing for the genetics section of the Botany optional subject.

4. "Economic Botany" by S.L. Kochhar: This book focuses on the economic importance of plants, including their medicinal, agricultural, and industrial uses. It covers topics such as plant breeding, biotechnology, and plant resources. It is a valuable resource for answer writing, especially for questions related to plant utilization and conservation.

5. "Molecular Biology of the Cell" by Alberts et al.: This book provides a detailed understanding of the molecular aspects of cell biology. It covers topics such as DNA replication, transcription, translation, cell cycle, and cell signaling. It is a recommended resource for those preparing for the cell biology section of the Botany optional subject.

In addition to these books, candidates can also refer to previous years' question papers and sample answers to get an idea of the format and structure of answers. Online platforms like EduRev provide access to a wide range of study materials, including notes, question papers, and mock tests for the Botany optional subject.

To summarize, the recommended books for answer writing in the Botany optional subject include "Plant Anatomy" by B.P. Pandey, "Plant Physiology" by Taiz and Zeiger, "Genes IX" by Benjamin Lewin, "Economic Botany" by S.L. Kochhar, and "Molecular Biology of the Cell" by Alberts et al. These resources, along with previous years' question papers and online platforms like EduRev, can greatly assist in preparing for the Botany optional subject in the UPSC exam.