My son loves Where's Waldo books.  We've found some similar ones (Dr. Who, Fairy Tales), but  they don't seem to have their own sub-genre or consistent keywords.

Anyone have suggestions for ones that are similar in difficulty to the Where's Waldo?

Gamebook Search and Solve - where you are asked different questions on each page, sometimes more than one. I don't see any posted right now, but they might work.