The Book of Discipline, now more commonly known as Faith and Practice, gives Friends guidance in matters both spiritual and practical, with reflections on both our faith and agreed upon procedures, known as Good Order. Every Yearly Meeting has their own document, but there is remarkable similarity between various Faith and Practices. From time to time a Book of Discipline is revised to more fully reflect the current reality of Friends.
Here is a link to the current Faith and Practice of Pacific Yearly Meeting: http://pacificyearlymeeting.org/fp/index.html
You might find it interesting to read historic Books of Discipline for comparison. Here is one such link for the 1806 version of Philadelphia Yearly Meeting’s Book of Discipline: http://www.qhpress.org/texts/obod/index.html
Quaker Heritage Press has published a comparison edition of nineteenth century Friends’ disciplines in America, called The Old Discipline. This can be purchased through Quaker Books from Friends General Conference (http://www.quakerbooks.org/the_old_discipline.php).
Most current Faith and Practices like to include the following statement from the Elders at Balby in 1656:
“Dearly beloved Friends, these things we do not lay upon you as a rule or form to walk by, but that all, with the measure of light which is pure and holy, may be guided: and so in the light walking and abiding, these may be fulfilled in the Spirit, not from the letter, for the letter killeth, but the Spirit giveth life.”
You might find it interesting to read this statement however in context for fuller understanding. The document can be seen at this link: http://www.qhpress.org/texts/balby.html