As we are entering several days of long davening, it seems to me that this topic might be relevant for our readers (and ourselves).
With regard to whether or not one may learn during the repetition of the Shemoneh Esrei, Rav Moshe Feinstein begins by citing the Magen Avraham (O.C. 124:8) who states that so long as the one learning is able to answer amen to the brachot of the shaliach tzibur, then there is no need to stop him from learning. However, he also cites the book Vavei HaAmudim who objects to people learning during the chazzan's repetition, as the more ignorant members of the community will learn from him that such a practice is permitted without reservation or restriction and will ultimately come to miss the chazzan's repetition.
Rav Moshe then notes that when there is exactly a minyan in shul it is forbidden to learn during the chazzan's repetition, since if the person learning fails to answer amen to the brachot, then there will be less than a minyan answering and there is the possibility that the brachot of the chazzan will be considered brachot levatalot.