I read in class 9 and I want to give prmo . Which book should I read?

Hello student

Cover all mathematics till class 10 first of all with special attention to geometry. Never let geometry loose! Not a chance.

Next as you are done with this try covering up entire algebra specially Series,P&C,Polynomials(Extremely Important),Complex Numbers.

Make sure you practice tons of problems and give more than an hour to improve your thought process on every problem you are stuck with.

Solve previous years papers of olympiad (One of the books have all of these papers included already).

MATHEMATICS OLYMPIAD-
1)Functionalequationby Prof. BJV.
2)Challenge and thrill of pre college mathematics.
3)Problem primer.
4)Gems (Inter-1, Inter-2, Junior-1 and Junior-2)by AMTI.
5)Non routine problem in mathematicsby AMTI.
6)Anexcursion in mathematics.
7)Mathematical Circlesby Dimitri Fomin.
8)Elementary number theory by devid burton.
9)Problem Solving Strategiesby Arthur Angel.
10)Hall and Knight.
11)The art and craft of problem solvingby Paul Zeits.
12)Mathematical Olympiad Challengesby Titu Andreescu.

https://www.askiitians.com/olympiads/regional-mathematical-olympiad/
please go through this