Website

• ==USACO Guide==
• AcWing
• CSES Problem Set
• AtCoder
• CodeForces

Courses

• ==Competitive Programmer's Core Skills(Coursera)==
• Competitive Programming Course - some fun questions
• CodeForces Edu
• Carnegie-Mellon ICPC Course
  • topic-specific contests in CF group

Books

• Competitive Programmer's Handbook
  • CSES Problemset(mentioned before)
• ==Intro to USACO(Bronze, Silver) - Darren Yao==
• Java, C++ versions
• Principles of Algorithmic Problem Solving - Johan Sannemo
• Competitive Programming 4 - Steven Halim, Felix Halim
• CS Guide