Bion introduced The Grid, which gives perspective on development of thought. It gives an account of emergence of thought from the unconscious to concious thinking and how defence mechanisms prevent that from happening in many individuals. This is well illustrated by examples. 'Looking for the truth' in a person is Bion's main theme and I think that simplicity expresses what psychoanalysis is about.
He is an 'independent' to his presuccessors such as Melanie Klein, and veers away from some concepts or just uses different terminology to describe the same thing, however, his book can be used to critically evaluate works of others, or help us think out of the box.