What is the best book for the basic knowledge of Microsoft Office's commands and functions?

Nowadays, Youtube had become the supporter for almost any tutorial.

Check MS Excel Magic Tricks by Mike Girvin

Like you want to learn for Average & Percentage etc.

Link is https://www.youtube.com/watch?v=c_ZJLJK5PjM

And Worksheets would be available at: 

https://people.highline.edu/mgirvin/excelisfun.htm