I'm looking for a book where two main characters have got magical tattoos. One guys name is Will and a girl by the name of Jess.
Could this be Clockwork Angel?
This book is the first in The Infernal Devices, one of the various sub-trilogies set in the Shadowhunters universe.
- Most of the main characters are Shadowhunters, or Nephilim. They have runes inscribed on their skin (which look very like tattoos, and are referred to thus by the unenlightened throughout the series).
- Two of the main characters are Will Herondale and Jessamine Lovelace. The latter is certainly called Jessie at various points in the books, and probably Jess. It is also possible that you are thinking of Jem Carstairs, another character.