I’m in the UK & I read this book as a child in 90’s, it was a collection of stories but not a huge collection. It was a picture book.

The main story I remember is a boy going into a slimey castle that had been taken over by a monster with tentacles. There was slime everywhere, he found the centre of the monster & it was a big eye & he stabbed it with a sword right into the eye. This killed the monster & covered the boy in slime. He crawled out of the castle & then washed himself off in a river. I think there was a friendly dragon there too.

Other stories in this were a ufo kidnapping a boy & a monkey having nightmares, construction robots shaped like dinosaurs & an artificial horse with a lever in its back. It was an odd collection of stories, not classic.