Looking for weird, timeless books for kids

My first grader loves Roald Dahl, John Bellairs, and The Phantom Tollbooth, all books that are exciting and entertaining but aren’t trendy or particularly modern. They also ask quite a bit from their readers (or listeners in my kid’s case) and use fairly advanced language and themes. Humor is also nice, but not necessary.

I’m looking for books that aren’t necessarily old but have a timeless feel. We listen to a lot of audiobooks but even ones I could read to him would be greatly appreciated.