Actually, Ziona relinquished possession of both the Sword of Lathander (First Light) and the book shortly after they first retrieved them. The party made some good contacts with the church during that time, although they did feel that their reward for finding the two artifacts was negligible.
Since that time however, Ziona still feels the light of Lathander in her generosity and feels compelled to tithe 50% of all her wealth.
As to Ziona's absence... Well, no party member is excused from MY WRATH!
Guess you'll just have to wait to find out what really happened. Many twists have occurred since the current point of the story.