At the beginning of the story, the man has no memory. Step by step he starts to recollect some details. He travels into a world and discovers that he created it. There is an antagonist (male), who tried to kill him and wiped his memory in the process, hence the memory loss.
I would be happy if anyone knows this book and can help with the name and author.
Edit: I know that Amber series by Zelazny answers this description, but it is not the book I am looking for.
Are you looking for Piranesi by Susana Clarke?
It begins with the main character living in a fantastic world, and as the story progresses he starts remembering things, and searches for the truth behind them. He does eventually find that his memory had been altered. I don't want to give too much away, because it's the sort of book you're trying to figure out as you go alongside the character.
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That should be the World of tiers series by Philip José Farmer.
It is a series of seven books.
Since English is not my first language, I found an English summary of the first book; maybe it helps you to remember:
His past a mystery and his present unbearably mundane, Robert Wolff is simply trying to buy a new house in Arizona when he stumbles upon a secret doorway through space and time and enters the World of Tiers. Made up of ascending levels of jungles, plains, medieval cities, and, at the top, a Garden of Eden, and populated by fantastical creatures, from nymphs and centaurs to merpeople and strange amalgams nonexistent on Earth, it’s beyond anything Wolff could have imagined in his previous humdrum existence. And when his youth is restored in the bargain, it seems he’s truly found paradise.
But there are dark forces in this new world, and Wolff is plunged into an epic quest up through the tiers, accompanied by Paul Janus Finnegan, another earthling, now known as Kickaha. Wolff’s journey to find Jadawin, the Lord of this world, will lead to answers about his own identity—and determine his fate.
Wolff and Kickaha will face off against feuding Lords—who hold the power to control private worlds of their own design—and the depraved Bellers. Devices originally created in the biolabs of the Lords, the Bellers are now conscious entities waging war on the Lords and their “pocket universes.” As they infiltrate the bodies of creatures throughout the World of Tiers and hunt down the Earth-born, the survival of all the worlds hinges upon the battle between the strangers from Earth and enemies disguised as their allies.
Here is an image of my German version:
