Good questions. My rule of thumb is the following, and it falls in line with the Yoga and Buddhistic teachings: most of us start out as 'works in progress,' if you know what I mean. Our minds are tumbling, following each and every distraction and fantasy — and to add to that, now we know this and reliably are becoming upset by this since we don't have the inner tools yet to overcome it. Therefore, many traditions start people off with breath awareness — it's a supposedly 'vipassana' method but really it is a combination of samatha and vipassana [the author of your book, Gunaratana, even calls the method he suggests 'samatha-vipassana'] .

Mindfulness of breathing supplies the meditator with the essential trainings of returning to relevant things (the breath) and away from things which had instinctively pulled the meditator for probably decades (fantasies, sensations, states, etc), also to gain a vipassana-like understanding about the nature of self-defeating striving (when the meditation becomes pleasurable, you will soon know the difference between goal-patience and obsessive-greed for that pleasurableness).

That said, there's another general note about your question which is mentioned briefly in the Buddhist scriptures, but head-on in the Yoga Sutras of Patanjali: when you're in a place in life where you're just totally overwhelmed by conditioning, seriously repeating harmful habits that don't respond to intelligence/teaching, then it's time for concentration-practice to stabilize the mind:

Patanjali:

The causes of suffering are not seeing things as they are, the sense of ‘I’, attachment, aversion, and clinging to life.

In their subtle form, these causes of suffering are subdued by seeing where they come from [vipassana]...

In their gross form, as patterns of consciousness, they are subdued through meditative absorption [samatha, which causes 'dhyana or jhana'].