Volcana hasn't appeared in a lot of issues, so each one is reasonably important to understanding her history.
Secret Wars I was the introduction to her character, and where she fell in love with Owen Reece.
Secret Wars II was probably her most important era. It showed her maturely dealing with the Beyonder as both friend and enemy, and her relationship with Reece reached its apex. This is probably her most deeply written storyline.
Her appearances in a few later random comics had her move on from this relationship. Fantastic Four 318 and 319 retconned the origin of Reece and the Beyonder's power, and Reece left her to become a Cosmic Cube. Marvel Comics Presents 88 has traveling to New York and fighting the Wizard as a solo hero, trying to move on with her life. In Fantastic Four annual 23, she similarly fights off Moonstone. In Fantastic Four annual 24, Reece returns and she gains new powers, but unhappy with his secrecy, she dumps him. This is her last major, on-panel appearance.
She-Hulk 10 (2005) presents her origin and friendship with Titania.