I want to buy a book on programming, which language should I choose.

Answer 1

Learnig Processing (easy OOP)
https://learningprocessing.com
Nature of Code (algorithms and simulations with OOP) free ebook by Daniel Shiffman, if you want, you pay as you like.
https://natureofcode.com
The material covered in these books is covered also in his video tutorials on The Coding Train
https://m.youtube.com/user/shiffman/playlists?shelf_id=2&sort=dd&view=50

https://m.youtube.com/user/shiffman/playlists?shelf_id=6&sort=dd&view=50

Language: simplified Java
Environment: Processing IDE
https://processing.org

Answer 2

For a language-independent approach, "Algorithms + Data Structures = Programs" by N.Wirth.

Answer 3

Depends upon type of application you want to develop
1) for desktop application start with C
2) for web application start with html & CSS
3) for mobile application first learn 1 or 2