There are an awful lot of excellent books in this thread!
If the chosen book/series is going to be read at school age, when most people study literature, I think Phillip Pullman's His Dark Materials trilogy would be a good choice. It has plenty of depth - the philosophical question of how to live your life which is at the series' core, the way the plot is used as an allegory for The Fall but with key ideas being flipped on their heads. In this sense it is like many older classic novels, but it is much easier to read. I think the combination of depth, readability and, of course, brilliant writing makes it an excellent series for educational use.
If the chosen book/series is going to be read at school age, when most people study literature, I think Phillip Pullman's His Dark Materials trilogy would be a good choice. It has plenty of depth - the philosophical question of how to live your life which is at the series' core, the way the plot is used as an allegory for The Fall but with key ideas being flipped on their heads. In this sense it is like many older classic novels, but it is much easier to read. I think the combination of depth, readability and, of course, brilliant writing makes it an excellent series for educational use.