Richard Foster is probably this age's best selling Quaker author and his books represent the best insights of the Quakerism that prevails in most of the world - not just the USA - although not in the UK.
This book, Celebration of Discipline, caught grief from both liberals and fundamentalists (within and outside Quakers). The liberals sometimes object to his biblical emphasis, and the fundamentalists sometimes object to his "political" advocacy of social concern as well as to his use of meditative disciplines. In fact, the inclusion of meditation in his writings (very congenial to Friends/Quakers) resulted in a brief campaign to have him excluded as a speaker from some evangelical events.
However, many of us appreciate his gracious, tender approach, his willingness to be self-disclosing, and his humor.