The following information is provided compliments of Gwen Gosney Erickson, Librarian and College Archivist of the Friends Historical College at Guilford College Hege Library in Greensboro, North Carolina.
The A to Z of the Friends (Quakers), by Margery Post Abbott, et. al. (Scarecrow Press, 2006)
Note: Rev. pbk. ed. of: Historical dictionary of the Friends (Quakers) (Scarecrow Press, 2003) which is even better.
The Quakers in America, by Thomas D. Hamm (Columbia, 2003)
Quaker Faith and Practice: The Book of Christian Discipline of the Yearly Meeting of the Religious Society of Friends (Quakers) in Britain [3rd edition] (Quaker Books, 2005)
A Living Faith: An Historical and Comparative Study of Quaker Beliefs, by Wilmer Cooper (Friends United Press, 2000)
The Friendly Guide to Quakerism (Youth Committee, Yearly Meeting of Friends in Ireland, 2004)
Quaker Writing: An Anthology, 1650-1920, by Thomas D. Hamm (Penguin, 2011)
Friendly Persuasion (excellent movie and novel by Jessamyn West which is good for those unfamiliar with Friends to see what things were like in the mid-19th century)
I Take Thee Serenity, by Daisy Newman (Novel with good descriptions of Quaker faith and practice set in 1970s)
Quotations which are used in the novel are taken from the following two books:
Practicing Peace a devotional walk through the Quaker tradition, by Catherine Whitmire (Sorin Books, 2007)
Plain Living, a Quaker Path to Simplicity, by Catherine Whitmire (Sorin books, 2001)
One-stop stopping for the items listed above and more at QuakerBooks
Pendle Hill Pamphlets which provide concise essays on a variety of different topics relating to Quaker practice and belief