"Christian spirituality" is slightly redundant; coupled with the adjective "nonreligious", it's absurd.
That's how I feel about Blue Like Jazz - the good parts are things I already know, and the rest is crazy. Donald Miller does his best to appeal to the hippie in us, but I think it's a failed experiment. Sure, it's fun to soliloquize about God while laying in a field and smoking a pipe, but I wanted something deeper than some guy's faux-philosophical musings. I didn't notice a conclusion to Miller's story either - perhaps a better title would have been "Scattered Thoughts On Watered-Down Religion, or (I'm Not A Theologian But I'll Talk About God For A Couple Hundred Pages)".
I can see why Blue Like Jazz would appeal to some people, but it wasn't my cup of (indie, fair-trade herbal) tea.