here i am suggesting some good books for GATE 2021 civil branch
1. for engineering mathematics - discrete mathematics and its applications by kenneth rosen , higher engineering mathematics by B.S grewal , advanced engineering by erwin kreyszig
2. for structural engineering - theory of structure by CK Wang , S.rammamrutham , theory of elasticity by timoshenko and gere , theory of structure by wilbur and norris , strength of materials/mechanics of structure by gere and timshenko , E.popove , L.singer , B.C punamia rammamrutham
3. for geotechnical - soil mechanics by gopal ranjan and rao , venkat ramaiha , S.K garg , B.C punamia , VNS murthy
4. water resources - water resources by SK garg, water and waste water by SK garg 1 and 2 , fluid mechanics - RK bansal and fox and richard
5. transportation engineering - transportation by justo khanna
for more details kindly go through the link
all the best!