My seventeen years as a children's librarian taught me that many children choose their books by subject rather than author or genre. Similarly, pet books are always in demand, as are the many splendid books on crafts. But children often do need adults to guide them to the right books and alert them to exciting topics. When Titanic was in theaters, books like Robert D. Ballard's Exploring the Titanic were instant hits as soon as children knew about them. Kids who hadn't thought much about codes suddenly wanted to send each other secret messages.
Some children choose fiction reading as well based on what they love.
Boys especially lean toward fiction on wilderness survival, sports and history. For them, outstanding novels like Gary Paulsen's Soldier's Heart about the Civil War and Walter Dean Myer's Fallen Angels about the Vietnam War bring history vividly to life. While demand for books about wizards, dragons and other magical characters has always been high, it skyrocketed when Harry Potter came on the scene. Experienced readers can locate more fantasies on their own, but most need guidance finding excellent series such as those that start with A Wizard of Earthsea by Ursula Le Guin and Dealing With Dragons by Patricia Wrede. Fascination with topics like magical creatures cuts across all age groups.