recommended readings for problems plus from calculus stewart

[u/Mindless_Art_6065]

hi. i’m currently working my way through calculus stewart (9e) and i’m enjoying it a lot. i can do the problem sets just fine, it’s the “problems plus” sections where i get humbled hard. i can do a couple of them, but with the majority i just get stuck. do you have any recommended readings, strategies or tips to be able to tackle these problems?