The Yshan Kings Saga — Michael Davies

There are three books in the Yshan Kings series and this is the collection of all of them in one volume. In Part One, the children of the Galactic Empire come to Earth to seek the Star of the Yshan Kings, a sentient crystal that has guided the Empre for 5,000 years. In Part Two, a peaceful, prosperous Empire is attacked and defeated by an alien race from another Galaxy. The Imperial Fleet escapes and hides. In Part Three, the new Yshan society which established itself in another Galaxy learns of the tyranny imposed on the home planet by the aliens with the help of a cruel Empress and sets out to rescue them.