The Clockwork Rocket trilogy is ++++, and Dichronauts is ++--. Reading these really helped my intuitive grasp of relativistic mechanics. Incandescence is also

The Clockwork Rocket trilogy is ++++, and Dichronauts is ++--. Reading these really helped my intuitive grasp of relativistic mechanics. Incandescence is also cool but (minimal spoilers) is more or less the normal +--- metric. Not nearly as mind-bending as ++++ or ++--.