Although there are only seven alphabetical letters used in music – A B C D E F G – these are repeated at higher and lower pitches over a large range of notes. Because everybody has a different voice range, not all singers will be comfortable with the same pitches.
The actual pitch of the notes given in the examples in this book are just a guide, so if you feel comfortable singing the examples higher or lower than indicated, that is fine. The important thing at the moment is to get the correct timing. Try this example which combines half notes and quarter notes.