General grammar and style fixes in the book

Contains numerous grammar, style and readability fixes (see diff)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | >= 2.3
| Fixed tickets | 
Same as before - I think the "or not" is helpful here. If we don't use it, we should change to use some usage of `if` instead of `whether`.