One of the most important aspects of learning to play any instrument is the development of a strong, consistent tone. You can often tell the difference between a good player and an average player by hearing them play a slow simple melody and listening to their tone.
A good player can really make their instrument sing and make a simple melody sound very moving. As you play the examples in the book, imagine you are singing the melody through your instrument and pay particular attention to your tone. Try playing at various dynamic levels (volumes). If you have trouble maintaining a strong, even tone at all volumes, practice playing long notes (at least 4 beats) on each of the holes of the harmonica at a slow tempo. In time you will notice a marked improvement in the consistency of your tone.