Book V, Gamadin: Core:

After surviving near death on the planet Gazz, the Gamadin crew discover Millawanda needs a tune-up big time or she will cease working altogether. The only choice left, according to Leucadia and Wiz's calculations, is to return Millie to her makers which, according to the ancient scrolls of Hitt, is a planet somewhere at the heart of the galaxy . . . the Galactic Core. It's a one-way trip if they don't find Millie's origin and the rarest blue crystal of all, which the old ones called "Aara," that Millawanda must have to survive. The journey is fraught with peril. Running on only a fraction of her power, Millie must chose her battles carefully. Wiz and the cats had been left behind while crossing the Cartooga- Thatt star desert, when they are attacked by ruthless killers known as "the Mysterians." Millie's course is not totally clear, either. The Gamadin crew must piece together a long forgotten route that no one has traveled for over 17,000 years! Who knows what they will find. . .