Mary Boyce is one of the foremost scholars on Zoroastrianism, and has written a few books that might be a good introduction. I think the first volume of A History of Zoroastrianism might be good - as well as Textual Sources for the Study of Zoroastrianism, so that you can get a closer look at the primary sources as well.

[Edit: I think Zoroastrians: Their Religious Beliefs and Practices is a summary of the first volume of A History...]

Mary Boyce is the main authority on this; however, I found her main book, History of Zoroastrianism, to be a bit disappointing (her chapters on the early period are in my opinion far too ambitious — our only source on the origin of this religion is a very obscure text written in a language that has been dead for 3000 years, so caution is needed when approaching it). However, it is also the most recent survey of the entire history of Zoroastrianism. Otherwise, if you are just interested in a short and accessible explanation, the Cambridge History of Religions in the Ancient World has a nice chapter on this, written less than a year ago by one of the leading specialists of the field. Cambridge Histories of Antiquity, generally speaking, can also give you interesting stuff on the relation of successive Persian polities to the religion (which was particularly important for the last great Persian power, the Sasanians). Introductions to translations of the Gathas (e.g. Humbach & Feiss, 2010) tend to be quite detailed and interesting (especially, of course, in their analysis of the very beginnings of the Zoroastrian faith).