Some nice descriptions of Piserchia books by Der Trihs from the Straight Dope message board (scroll down):
Doomtime: Set in a far future Earth where the dominant life forms are a pair of mountain-tall sentient trees that have extended their roots over much of the planet. They hate each other, and are constantly trying to destroy one another. They can temporarily (or permanently) absorb creatures into themselves, including people; this process lets them read people's minds, and is pleasurable enough to be addictive, which is one way they control people.
Spaceling: The protaganist is an orphan girl who is a mute, a person who can see the Rings, which are drifting rings of light that are interdimensional gates to those who can see them. Anything or anyone that passes through is transformed into a new form. She, however, secretly has abilities that others don't. She can see many more Rings than others can, her forms are different than normal people's forms, and she can command the Rings to move as she desires. She ends up having to discover her truly odd origins, and deal with multiple conspiracies that want to help/hurt/use her.