As a reader I love books that challenge my intellect. I read a lot of historical fiction and the more real history I learn the more I love the book. I read lots of mysteries and suspense. Nothing keeps me turning pages like the clever weaving of clues and connections. Science fiction and fantasy? Create a world or science that sounds plausible and stays in my thoughts for days or months until your next book comes out.
Even better, introduce me to a new word, not a big show off word, but a rarely used word that is perfect for the scene. I have a little spiral notebook that I write down words that I like and intend to use one day when they fit perfectly. Whatever you do, don't dumb down the prose to fit me as a reader.
People who read books are smart. If a reader opens a book expecting a historical mystery in the time of King Henry VIII, keep the facts straight and teach me a little history woven within the plot. If you're writing science fiction, give me a little real science to make the fictional part more believable. Challenge the reader whenever you can. Never underestimate the intelligence of the people who might read your books.
Have you ever put a book down because it insulted your intelligence? Have you ever read a book where you felt the author was showing off by using big or obscure words when a simple better word would do? Do anything special for this longest day of the year here in the northern hemisphere?