This feature will not be in chronological order (based on reading age) as it continues, but it makes sense to start at the beginning. The earliest mind opening books I can think of were by Dr. Seuss.
The good Doctor’s writing, in general, is a great place for early stretching of the brain. Following rhymes and patterns are some of the first puzzles kids are able to solve. This can be either guessing what word or phrase is coming up, or making up rhymes on their own. The ability to rhyme like Dr. Seuss not only exercises the grey matter, but is a wonderful, yet harmless way to annoy people as well (and apparently frighten my Uncle, but the less said about that the better.) Cultivating the proper level of smart assery in the face of authority is an important component of expanding one’s viewpoints.
Both of these start with plain old (if poetic) reality, and then leap off the cliff of rationality, getting more and more distant and deranged with each turn of the page. The rhyme and meter hold constant, as the subject matter gets more bizarre. This lets the form of the book hold the reader’s hand reassuringly, as the content takes them diving off the deep end.
Since this is the very first of the series, a bonus mind opening book for the same age group:
The Sesame Street classic The Monster at the End of this Book
This one provides a nifty introduction to a couple of devices that come into play later in a strange thinker’s life. Grover’s paranoid romp is a great first look for the preschool set at both self reference and breaking the fourth wall. Both of these are key elements that will creatively mess with their heads in later mind expanding reading.
Keep an eye out for future editions of Jeff's Books to Open your Mind...whenever I remember what they are.
(I said "open" not "improve" your mind.)