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For the moment your focus is on using these three constructions to describe the duration of some action or state that began in the past and continues to the present. As Julvenzor and Rusty have indicated:
Hace una semana que estudio español. =
Estudio español hace una semana. =
Estudio español desde hace una semana. =
I have been studying Spanish for a week.
Just as your book states.
However, the first two of these constructions can also be used to describe when some action occurred and completed in the past, measured as the time span since the event occurred or completed to the present.
Hace una semana que estudié español. =
Estudié español hace una semana. =
I studied Spanish a week ago.
The third construction doesn't allow "estudié español" as the conjugated verb phrase: it can't be used to say how long ago something happened, it can be used only to say how long ago something started.
No estudio español desde hace una semana. = I haven't studied Spanish for a week.
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