Apprising Ministries is pleased to point you to this excellent and concise review of the classic book which spawned The Cult of Guru Richard Foster. Gilley says:

Written over twenty-five hears ago, and proclaimed by Christianity Today as one of the ten best books of the twentieth century, the influence of Celebration of Discipline is all but incalculable. Foster is a Quaker, so his spiritual life is grounded in the subjective “inner light” presupposition of the Friends... Overall Foster’s book is an encyclopedia of unbiblical teaching,... (Online source)

You can read the entire review from Gary Gilley of Celebration of Discipline by Richard Foster right here.

See also:

DO YOU KNOW WHERE YOUR MYSTIC TEACHING COMES FROM: RICHARD FOSTER

CONTEMPLATING THE INNER LIGHT OF THE QUAKERS

CONTEMPLATING THE INNER LIGHT OF THE QUAKERS (PT. 2)