The source of all things Zentangle is http://www.zentangle.com/
One Zentangle a Day by Beckah Krahula – fantastic and systematic introduction to Zentangle.
Tangle Patterns and so many other Tangle related resources http://tanglepatterns.com/about
Zentangle untangled by Kass Hall – nice overview but particularly nice descriptions of use of different art materials (read color) in Zentangles and ZIAs.