Git is a free and open source distributed version control system designed to handle everything from small to very large projects with speed and efficiency. (Git)
The gate to the open source world is git so it's one of the most essential tools that any developer should learn.
The Pro Git book is one of the best books in the mattered subject. One can find some notes in this docs.
Solutions to the coding problems in the HackerRank website.
HackerRank Profile
Solutions to the coding problems in the LeetCode website.
LeetCode Profile