Piers Anthony's Cluster Series Volume 2 - Chaining the Lady
Description from the back of the book
Ten centuries after the War of Energy, in an embattled solar system, Melody of Mintaka - direct descendant of Flint of Outworld - fingds that only she can save the Milky Way Galaxy. Victory by the Andromedan spacefleet seems inevitable, for they alone control a mighty new weapon - the ability to invade other boxies and dominate them utterly.
The fate of the Cluster dimension hangs in the balance as Melody assumes the forms of alien and animal, male and female, to turn the tables one her captors. And the stakes are awesome, for the Andromedans' strange power is the key to a Pandora's Box holding the very secrets of the Universe....
Condition - yellowing pages due to age (it's 41 years old) but otherwise very good with some general signs of wear.