The Holy Grail of Eris, Vol. 4 (light novel)

Lucia O'Brian is heading to the Republic of Soldita, and Connie, Randolph, and a certain glamorous ghost wind up coming along for the ride. At first Scarlett doesn't think much of the idea, but all that changes once she learns their destination—the island of El Sol, where her mother once told her the starry crown of Cornelia Faris lies hidden. But there's more to this treasure hunt than meets the eye, and plenty of trouble waiting for Connie and Scarlett across the sea.