This little red booklet is the reason why I am a "Quakerly-inclined" Unitarian. Each of its forty-two paragraphs is a challenge, a comfort or a reminder of some fundamental religious or spiritual truth. I first came across it about 15 years ago, and was blown away by it - it really speaks to my condition (to use a Quakerly phrase).
Reading it has made me realise that Unitarians and Quakers have an awful lot in common, both theologically and spiritually. Take the introduction: "Within the community there is a diversity of gifts. We are all therefore asked to consider how far the advices and queries affect us personally and where our own service lies. There will also be diversity of experience, of belief and of language. Friends maintain that expressions of faith must be related to personal experience. Some find traditional Christian language full of meaning, some do not. Our understanding of our own religious tradition may sometimes be enhanced by insights of other faiths."
You could build an entire service around each paragraph ... over the years, I probably will.