I'm sure I'm going to catch a lot of flak for this one;
The Lord of the Rings series.
I tried reading these when I was younger and was bored out of my head when Tom Bomdabom (whatever) showed up. All the "songs" that they sang in it were boring as hell too.
I tried again when I saw the first movie and was all "Jaysus these are boring".
But I'm sure that there are plenty other movies that were better than the books they were based upon.