Dietrich Bonhoeffer's Letters and Papers From Prison and The Cost of Discipleship are way up there for me, though not straightforward autobiographies in the sense that Merton's is. Bonhoeffer's notion of "cheap grace" as opposed to "costly grace" is as pertinent as ever in the age of cut-rate TV evangelists.

I also admire the spiritual writings of Dorothy Day and the story of the Catholic Worker movement (without, alas, being able to recall the name a particular book). And two great novels that amount to spiritual autobiographies of a fictional character: George Bernanos' Diary of a Country Priest and Willa Cather's Death Comes for the Archbishop.