If we’re talking old authors & books I recommend Doris Piserchia (she only died in 2012, but she didn’t publish anything after 1983 due to a death in the family).
Spaceling: A story about an orphan girl with the ability to enter other worlds and shift into alien bodies by stepping through floating “rings” visible only to those like herself. She herself is apparently (at first) unique in both not following all the transformation rules others do, and in being able to see many more rings than others with the same ability. Exactly why this is and discovering the truth involves plenty of adventure, intrigue and general weirdness (and now I want to find it and re-read it…).
The Dimensioneers: While it shares a few ideas with Spaceling (orphan girl, can enter other worlds) it’s definitely its own thing. With her mutant lion bond animal he can enter another dimension that in turn allows access to a “great ring” of worlds, many of which are inhabited; and at least one is on a conquest spree.
Doomtime: Set in the distant future when Earth is dominated by two mountain-sized sapient trees whose roots underlay most of the world and are eternally at war with each other. One weird detail is how it is possible to “dip” into the trees, temporally merging with them and communicating mind-to-mind with them.
The Spinner, The Fluger: Both different sort of “monster” stories. The Spinner is about a sapient predator from another dimension that covers the city it arrives in in living webs that slowly consume anything put in them (like people), and intends to start breeding soon…The Fluger is about the Fluger of the title (a nigh-indestructible sapient alien predator (the thing withstands artillery fire) that is terrorizing a future city), and the alien assassin hired to kill it.
A Billion Days of Earth: Set very far in the future when humanity has evolved into psychic “gods” literally living in the sky, while the surface is inhabited by the sapient descendants of animals. Presumably uplifted by humanity at some point in the past; an interesting detail is that none of them have actual hands, they wear artificial mechanical hands like gloves over their paws. Presumably originally designed and built by their creators as they couldn’t have done it themselves. The story kicks off when an entity calling itself Sheen arrives, a silvery amorphous being that can “eat the egos” of people, engulf them and turn them into puppets.
The Deadly Sky: Her last novel, it’s set in a future city near a hole in the sky from which invaders from another dimension are attempting to turn into a portal for invasion while the city continually tries to stop them. There’s a dangerous maze filled with traps inside the rip which has to be navigated by the defenders of the city; they end up losing more and more body parts to the traps which are replaced with glass cybernetics, eventually ending up as entirely glass “drells”.