Love - the entire trilogy is about love.
There are some themes about love that run throughout the trilogy:
love of/from family
love of/from friends
But each book also has a theme about some nature of love.
The Unconventional Affair - an overcoming love, a love that is transcendent
Tangier - an empowering love, a love that makes us better
Tough Cookie - a sacrificial love, unconditionally giving of ourselves for someone else.