You are likely to be familiar with using Ctrl+F to quickly locate where a particular string occurs. grep is similar, but much more versatile and feature-rich version of the search functionality usable from command line. Two different implementations - GNU GREP and RIPGREP are discussed in this book.
The book heavily leans on examples to present features of grep as well as regular expressions one by one. It is recommended that you manually type each example and experiment with them. Understanding both the nature of sample input string and the output produced is essential.
Exercises are also included to test your understanding.
You should have prior experience working with command line and bash shell, should know concepts like file redirection, command pipeline and so on.
For a preview of the book, see sample chapters at https://github.com/learnbyexample/learn_gnugrep_ripgrep/blob/master/sample_chapters/gnu_grep_sample.pdf
Visit https://github.com/learnbyexample/learn_gnugrep_ripgrep for code snippets and files used for examples/exercises in the book. The repo also has solutions for exercises.