To date, there are three volumes in the True Tenchi Muyo! series of light novels. Volume 3 "Washu" was a turning point in the level of detail that co-authors Kuroda and Kajishima wanted to bring to the main series.
Together they build a steady pattern of pranks and outsmart some major threats in the known universe. This 1999 novel completes a trilogy.
For more information see continuities.
The man burst out laughing. "New student? Do you have any idea where you are? Only the chosen amongst the most elite in the entire galaxy can enter here. It's not a place for a child like you. Perhaps you're mistaking it for the Department of Ancient Literature the next town over." The man looked at her with utmost contempt.