I think this book was written in the late 90s to early 2000s by a male writer.
The detective is hired to find a guy who he eventually finds. Maybe the guy he was looking for was a scientist? The detective gets into trouble along the way and is saved by cats. He somehow wakes up at a hotel run by cats. Regular cats. He wonders how it gets maintained. I think he has a cat.
He has a friend who somehow gets lost in a dangerous part of this alternate world and his friend is killed by a memory of his own mother come to life.
The end of the book is when the reader finds out the world as we know it exists and this is an alternate world that he or maybe someone else (I don't remember) saw the landscape of from an airplane.