Pendle Hill Pamphlets
In addition to books on Quakerism, the Pendle Hill Pamphlets series offers a tremendous amount of information in small packages. They started publishing in 1934, and just released pamphlet #382, Holding One Another in the Light. A number of pamphlets are available for free online, while most can be ordered from the Pendle Hill Bookstore's catalog.
Pendle Hill, home of the bookstore and pamphlets, is "a Quaker Center for Study and Contemplation", located in Wallingford, Pennsylvania. It's named after Pendle Hill, on which George Fox wrote that "the Lord let me see atop of the hill in what places he had a great people to be gathered." (Nickalls, 103-4; QR, 70; QS1, 78-79; QS2, 30 )