The Quaker Reader (1962, 1992)
The Quaker Reader was one of the first books I bought on Quakerism, mostly because it was the only book on the subject my local Borders had in stock. It proved a great choice.
Pendle Hill Publications reprinted this 1962 book in 1992, updating the introduction slightly. Sixty-one separate entries explore Quaker faith and history. One of my favorite features is that it includes pieces from non-Quakers, and not always friendly non-Quakers, as well as identifying Quakers from the early period, Quakers who were born into the commmunity, Quakers who were convinced, and a few Quakers who left.
If you're looking strictly for religious insights, this may not be the right book. However, if you want a sense of how those religious insights related to how their proponents lived, it's an excellent book.
[Sorry - I had to turn comments off for this post because of an amazing volume of spam.]