Which books to refer to for Geography Optional?

There are actually 8 books that cover almost 80-85%
of the syllabus.

It is updated, enrichment of deeper conceptual clarity that requires more books and coverage of current affairs based questions. Sincerely managing GS goes a long way in helping geography too.

Basic Books

1.  NCERT - XI-XII

2. A certificate course in geography by Goh Leong.
3. Made simple series by Rupa publications vol-1and  11 ...

Reference Books :
1. Indian Geography by DR Khullar
2. Human Geography by Majid Hussain.

3. Models and theories by Majid Hussain. 4.Geographical thought by Dixit.

5.  NCERT Class X  textbook, (old edition) is good for economy Geography coverage. It will also help GS geography topics like World industries, world resources, trade & distribution...
6. Geographical thought by Sudipta Adhikary (A good supplement for thought topics)
7. Physical Geography -By Ravindra Singh (if one wants to avoid specialized books for Physical Geography syllabus.
 There are 5 specialized books by Savindra Singh that have certain specific chapters that are not covered through the above book.

Coverage of above physical Geography text Book covers almost 80% of  the physical geography syllabus) for the other topics of the Physical geography syllabus, one has to refer to about 2-3 chapters from each of  the below books:

One can take photocopies of the selected chapters from libraries and study rather than buying these books.

1.Geomorphology - By Savindra Singh 2.Oceanography - By Savindra Singh.

3. Climatology By Savindra singh.

4. Environment Geography By Savindra Singh

5. Biogeography By Savindra Singh

8. Settlement Geography by K Siddhartha...
Other references:

Like in all other subjects, and in GS, the student must-read by various magazines and current affairs based enrichment. Some of the sources one can be regular with are:

1.Yojna
2. Kurushetra
3. Geography and you 4.Select Un docs on population, development and environment.

(UNDP is most helpful).

5. NITI aayog reports ( particularly, those on Agriculture, Infra, Regional development, Rural development, etc).