This is a difficult book to read but not because of the difficulty of the subject matter or because Gabriel Marcel is abstruse like a Sartre or Heidegger. Rather, Marcel is difficult to read because he is very tangential. He veers off topic frequently into imagined objections to or issues about what he is presenting--objections and issues whose pertinence often seems inflated, trivial, or irrelevant.
Almost as disturbing is Marcel's frequent habit of extensively referencing his other philosophical works and plays. While Marcel does mention other thinkers (e.g. Sartre), he does so mostly in passing. One gets the impression that he used the Gifford Lectures to market the oeuvre of Gabriel Marcel. Given that he delivered these lectures in the philosophically rich post World War 2 era (1949-1950), Marcel's work in volume 1 chronicles a disappointing missed opportunity for philosophical engagement.
None of this is to say that Marcel doesn't have his moments. He does, but, unfortunately, they are merely that: moments. His discussion of the human body is a case in point. He makes the interesting and potentially seminal claim that our sense of ownership of secondary and tertiary things arises from our felt ownership of our bodies. But he doesn't develop the insight with any depth because he launches into other considerations, ones (like the body should be understood as a subject and not an object) that would have served to progress the discussion of ownership had he presented them earlier in the discussion.
I found volume 1 so disappointing that I now have serious reservations about reading volume 2.