Source: Statistical Sleuth in R, Chapter 7, 8, and 9
How to access: This is accessible from the link above (free).
Contents: This covers the simple linear regression (Chapter 7 and 8) and multivariable linear regression (Chapter 9). The text is written in R Markdown. The R code and the result are displayed together, which would help readers to learn how to do it with R. No math formula. This would be helpful to grab the idea of regression.
Example(s) used in this: Distance and radial velocity among extra-galactic nebulae; Brain weight, body weight, gestation length, and litter size of mammals; and Amounts of light and the growth of meadowfoam
Prior math/stat knowledge to fully understand materials: This is an introductory level and no prior statistical or mathematical knowledge would be required.