Actually, there are little to no primary sources regarding Pre-Roman Britain, since no written sources were left. Most of what we know is based on excavations, and ironically on testaments written by the likes of Caesar detailing how the various tribes of Britain lived at the time Romans became actively involved (first by trading, later by conquest). The first book I would recommend to read is Commentarii de Bello Gallico (Gallic Wars) by Julius Caesar himself, since it provides valuable insight on the briton's way of life and social hierarchy in some sections.