mm, i agree
i've just started reading age of reason, and you're very correct about the "certain mood." it's definitely not a book to make you smile, but although a strange pensive contemplativeness permeates every page, it's not overly sentimental or overtly tragic. then again, i'm only on page 60.
when i read camus' "the stranger" i was struck by the same pervading sense of...not quite unhappiness...but a certain sort of feeling that (apparently) i'm unable to describe.