Hello,


First complete NCERTs of class 11 and 12 and make notes out of it.Remember, never skip NCERT.
Then for paper 1, there are two books that most toppers suggest one is Robert baron and the other is Saundra k Cicarelli. You can pick anyone of them. If you are a slow reader then go for Cicarelli.
Now for Paper 2 you can go for Smark Swain.
All the books are available on Amazon.
Other than that, watch Paul bloom videos on YouTube, Crash course psychology series video.
Follow Psychology today website and join Telegram channel name Psychology Optional for UPSC CSE.
If you want notes then you can buy Mukul Pathak notes.There are many toppers who have uploaded their notes and answer copies, one is Anurag jain and you can follow is blog.

Good Luck

Hii aspirant
Hope you are doing well amid covid-19 pandemic
Psychology is an interesting subject of you are able to build your interest in the subject then it becomes easy otherwise it can be tricky as well as it contains lots of theories as well as case studies
As per the perspective of UPSC emphasis is basically on the core areas of psychology
For this you should definitely consider
NCERT BOOKS :- as they provide you with the best insight of the subject and covers vast areas of the field, not only this it also give you overview of the subject, now with better understanding of the subject with the help of NCERT books you can go for some books like

• Social psychology:- Baron and Branscombe
• Applied psychology by Smarak Swain
• Fundamentals of cognitive psychology by Ronald Kelogg
• Atkinson and Hilgard - Psychology
• Theories of personality
• Social psychology
• Abnormal psychology
• For statistical part AK Singh is best

these are some of the books as per my knowledge you can further explore for options and see videos and websites for more information
For pattern and syllabus for exam I'm sharing a link with you please go through it

https://www.google.nl/amp/s/competition.careers360.com/articles/upsc-psychology-syllabus/amp

hope it helps