Choosing a recommendation for the Summer Reading List was a struggle for me, because I don't really have favorite books. I don't really have favorite anything, for that matter. If you ask me my favorite movie, I will rattle off a list of dozens of titles. However, the reason I don't really consider any book a favorite is because I so rarely re-read them, whereas I will re-watch movies (in some cases dozens of times). I did have favorites as a child though. Specifically, my favorite book was Millicent the Monster, and I would very much like to recommend it to anyone who has a moody little girl. Sadly, it is out of print, but there are some used copies for sale on Amazon.
Among the few books that I have re-read in my life are the Madeline L'Engle books A Wrinkle in Time, A Wind in the Door, and A Swiftly Tilting Planet. Most people probably read them when they were younger, like I did. It's worth reading the books again as an adult though. They are every bit as good as you remember them. And if you didn't read them as a kid, then you definitely should read them as an adult. I have never read Many Waters, a later book in the series, but I plan to rectify that soon.
These books meant so much to me as a child, and so many things - scenes, bits of dialogue - about them stayed with me over the years. That's why I chose to re-read them. I wanted to know just how good my memory was. Everything was there just as I remembered it. That's how vivid these books were for me. It's a good reminder that great writing is not limited by the age of the intended audience. There doesn't need to be a sliding scale for quality.
However, if you are looking for something more adult, or you have already re-read the Madeline L'Engle books, I really enjoyed Gentlemen of the Road by Michael Chabon. He notes in the afterword that the original working title of the short novel was Jews with Swords, which is pretty apt. It's a historical fiction/adventure novel about the Jewish Khazars. One of the things I enjoyed so much about this story was that the Khazars were a people I knew absolutely nothing about, so in addition to getting a really fun read, I learned something new.