Book suggestions for an adult who struggles with reading?

My partner is 21 years old and trying to improve his reading skills. He was never taught to read by his family or schools growing up, and now as an adult has the reading level equivalent to a 7-8 year old.

We've gone to book shops and libraries to look at books for him, but the problem with books aimed at children is... they're quite childish. They're not something he would enjoy to read and, honestly, I think he feels a bit embarrassed at the concept of reading books obviously aimed at children.

He's a big fan of the horror, psychological and thriller genres, or really anything that's a bit dark or suspenseful.

If anyone has any recommendations at all it would be greatly appreciated!