This is a hefty book. Good libraries will have it.
The essay on Plotinus, which is by Armstrong, is the single, best, clear and synoptic account of that very difficult author that you will find in English. It is also very short, about 80 pages or so. Read it.
The rest of the book (which Armstrong also edited) can be ignored. I am not an admirer of Pierre Hadot, whose works have recently been translated (and written, for that matter) en masse -- His wife, Ilsetraut, was a much better scholar.