I'm a big fan of the "Very Short Introductions" series from Oxford University Press. They're usually brief and gives a wide starting point to a lot of different viewpoints of a subject, useful as a starting point into a topic. Seems there are about 730 books published in the series so far, with a wide range of topics. https://en.wikipedia.org/wiki/Very_Short_Introductions I personally haven't read it, but there is one for religion as well, "Religion: A Very Short Introduction" (9780190064679). Then there are a bunch of them around specific subjects in religion as well, browse the list in the Wikipedia article and you'll find them. I've read maybe ~20 books from the series and all besides two or three have been of utmost quality, so probably this one is at least a good starting point.
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